She has woken to a series of text messages sent the previous night by Thomas, her 65-year-old stepfather and the grandfather figure - "Opa" - to her children Sam, 11, and Amy, 13. The day before, she and Nick had agreed to let the kids leave straight after school to spend the weekend with Thomas and his wife Sarah - Maria's mother - at their waterfront home a little over an hour's drive away. Weekends with the grandparents weren't unusual, but they'd recently become an almost weekly event, leading Maria and Nick to start politely saying no. More time with their grandparents meant less time with school friends, bike rides with the kids next door, birthday parties and sleepovers.

Thomas had always doted on Sam and Amy, showering them - especially Amy - with expensive presents, picking them up from school, ferrying them to McDonald's and on ice-cream runs. Weekends away with Opa in his caravan, and on his fishing boat, had been regular pastimes since the children were small. Whole summer days passed tubing off the back of Opa's boat, warm nights were spent sleeping under the stars in the annex of his caravan. "Bring your friends," he'd often say.

Lately, however, Maria and Nick had noticed Thomas becoming strangely persistent: showing up unexpectedly at the house or the kids' school, appearing on the sidelines at sports carnivals they'd never mentioned to him, usually without his wife. When Maria and Nick began turning down his requests for another weekend away with the kids, Thomas would assure one that the other had already agreed, pitting them against each other.

And now, on this February 2013 morning, in these disturbing texts, Thomas claims he has discovered that Amy is "having sex with a 20-year-old man" - something Maria knows in her bones is an outright lie. "Do you want me to talk to Amy about it?" Thomas asks. Maria assumes the messages are a twisted reference to the events of the previous afternoon, when Maria had been called to the principal's office at her daughter's high school. The year 7 student and a friend had been sprung arranging to meet an older teenage boy in the school car park after the final bell.

Maria knows she'll need a serious chat with Amy when her daughter returns from her Opa's place, but after waking up to these text messages, she decides it cannot wait. What would make Thomas say such a thing about Amy? Something's not right. "I want to go and get the kids right now," Maria tells Nick.

Thomas and Sarah are standing on their waterfront patio, arguing furiously with Maria and Nick, who have arrived to be told they are not allowed to take their own children home. Through the glass sliding doors, Maria can see that Amy and Sam are hysterical, running back and forth through the open-plan kitchen. She's never seem them so distressed.

Thomas has told them a cruel lie: that their parents are getting divorced and that they are to blame. Thomas looms tall over Nick with a raised fist and a protruding beer gut, his heavy frame still carrying remnants of the muscle he wore in his younger years as a bouncer. "You're not taking the kids," Thomas says. He's screaming lies: that the young parents have been hurting their children, that Amy and Sam are not safe with them. Sarah is holding back Maria, calling her daughter cruel names. None of it makes any sense.

Maria calls out through the kitchen window, telling her children to grab their belongings and run through the front door to the family's white Subaru. They pile in, the children still crying as they drive away. It's the last time Amy and Sam will see their grandparents - or so Maria and Nick believe.

Putting a halt to any further visits, Maria finally has the mental space to assess things. So many moments with her stepfather over the years, once dismissed as insignificant, now stand out. Maria was just 14 when he came into her life as her mother's second husband. Born the eldest of four children in 1940s Germany, Thomas had moved to Australia with his family at the age of six. The family settled in Wollongong, where at just 19 Thomas married his first wife, with whom he had a son and a daughter. The marriage ended in the 1980s, and Thomas, by now a mineworker, eventually made a new home with Sarah. At 39, Sarah herself already had three daughters, including middle daughter Maria.

There's one question Maria can't shake from her mind: is Thomas really a doting grandfather, or something else?

At first, Maria didn't warm to her new stepfather. Unlike her dad, who divorced from Sarah in 1986, Thomas was a bit of a yobbo, big and loud. He chain-smoked and he drank. He did things her own father would never do, like walk around the house naked, and remark on her figure and those of her friends. As a teenager she flinched when he slapped her on the bum. But it was the 1980s and although she found it creepy, it wasn't the sort of thing you complained about. Maria recalls the way people would say, "That's just Tom."

Now it's 2013, though, and there's one question Maria can't shake from her mind: is Thomas really a doting grandfather, or something else?

"There's always been something off about him," Nick says when she raises the subject with him. There was the time Nick unwittingly unlocked Thomas's phone to find a photograph of another of his teenage granddaughters in a bikini. "I said, 'What is this shit?' He said, 'What? She's a beautiful young girl, she wanted me to see her new bikini.' After that I started watching him like a hawk, but obviously not good enough."

Maria decides to call her stepbrother Patrick, Thomas's biological son by his first marriage. The two half-siblings were never close growing up, despite there being less than two years between them. These days, living more than 300 kilometres apart, they rarely catch up, but Patrick is one of the few people Maria can ask about what she's thinking.

Far from calming her growing unease, Patrick tells her that he and his two daughters ceased contact with Thomas more than a year ago, after Patrick discovered that Thomas had been plying Patrick's two girls - then aged 12 and 14 - with marijuana and alcohol, and trying to coax them into taking nude photos of themselves. Maria can't understand it. How could Patrick have kept this from her - and why didn't he report it to the police?

Her worst fears are realised when Patrick mentions a photo of Amy that his daughters claim to have seen on Thomas's phone. Appalled and frightened, Maria contacts family services, supplying them with everything she can think of. But she realises with a sinking feeling that she has no real proof of anything, just a gut feeling that something isn't right, and a handful of texts and reports that suggest strange behaviour. Any time she peppers Amy and Sam with questions, they brush her off.

She's eventually contacted by someone from a joint child abuse investigations team, a group of experts drawn from NSW departments including family services, the police and the Director of Public Prosecutions' office. They begin by interviewing Amy, but Maria's 13-year-old daughter is reluctant to talk. She quietly insists that nothing inappropriate ever happened with her grandfather. There's little more that Maria and Nick can do; no sooner does it open, than the case against Thomas seems to close.

"Amy needs you." It's June 2015. Maria and Nick are enjoying a quiet night in front of the TV when Daniel, the boyfriend of their now 15-year-old daughter, enters the living room. Maria can tell by the scared look on his face and the way he speaks that something awful has happened. She hurries past him and into her daughter's room, which - with cream walls, a single bed and a computer atop a corner desk, and a little calendar on which her homework deadlines are marked - could be that of any teenage girl.

Amy is sitting on the edge of her bed. The skin around her eyes is red and swollen; she's clearly been crying for a long time. Maria sits down beside her and takes her hands, imploring her to reveal what's wrong.

Two years have passed since Amy assured the investigations team that Thomas never did anything to hurt her. But tonight she's told Daniel a different story, opening up for the first time about the years of sexual abuse she suffered at the hands of her Opa. With Daniel's support and urging, she's finally ready to tell her mother.

"I lied," she blurts out. "He has done things to me." The next few minutes are among the worst in Maria's life, as she learns there was an awful truth behind her fears. Not only that, but the abuse has continued - via secret texts and meetings initiated by Thomas - in the two years since she and Nick thought they had stopped the children visiting their grandparents.

Within minutes Maria is on the phone to the local police; an hour later, she and Amy are at the station, where Amy is taken into a private room and interviewed. "I'm going to have to refer this to Sydney," the policewoman explains to Maria. "We just don't have the resources to deal with it here."

A few hours later, Detective Senior Constable Jane Prior, a seasoned Sydney-based detective with more than 12 years' experience investigating sexual assault, receives an early-morning call from her superior.

"Pack a bag," she's told. "You're heading down the South Coast."

The next day, and in numerous, subsequent interviews, Amy sits down with Prior to tell her story. At first she's withdrawn, not psychologically ready to express what she lived through. But she slowly opens up, explaining that it all began when she was a toddler, when Thomas began touching her inappropriately and blowing hot air between her legs. She tells Prior of the first time he had "full sex" with her: it was 2009, she was about nine years old, they were in his white Nissan Patrol. She describes the strong prescription medication he gave her every time he saw her, so that she would not say "no" or feel any pain. Her Opa, she tells Prior, liked to treat her as if they were "dating" .

The abuse was so constant, over such a long period, that she has difficulty remembering specific days or incidents because he had "always done it". She hasn't, however, forgotten the room in which a lot of it happened, a children's cubby in the roof of Thomas's house which he custom-built when the kids were little. It was here that Thomas filmed Amy and Sam in various sexual poses and acts.

Forensic crime-scene investigator Ian Bennett is the first officer inside the cubby, the day a search warrant for Thomas's house is executed. Up a narrow ladder and through a trapdoor in the garage ceiling that Thomas usually kept locked, he discovers everything Amy described: mattress, lighting, cameras. Bennett has seen many grisly scenes in his seven years on the job, but nothing like this. He and his team seize a memory card from Thomas's car that contains three pornographic films depicting Amy and Sam. Police investigators are confident there would be many others. The alleged abuse is so immense, so twisted, that Bennett and Prior are not alone in wondering how it could all be true: the drugging, the films, the tunnel to a secret cubby in the attic of a waterfront home.

Six days after Amy tells Prior her story, her by now 13-year-old brother Sam tells his mother that he, too, had been preyed on by his Opa. "I remember feeling very nauseous and laughing ... in a way that I wouldn't normally," he later told a court of the first time he smoked a joint in his Opa's car, aged nine.

As Amy and Sam grew older and more likely to question their grandfather, he developed a new set of tricks to manipulate them, based around a trio of people who didn't exist. Bob, Lyn and Toey, purported friends of Opa, would send Amy and Sam texts, threatening to send the video clips they'd already made to friends and family if they didn't participate in more. The three characters would frequently argue among themselves, or insult one another. They were such believable and compelling personas that it would be 12 months into the investigation before police were able to convince the siblings that Bob, Lyn and Toey weren't real.

When Thomas swings his LandCruiser into his driveway on June 19, 2015, Prior and her fellow detectives from the NSW Child Abuse and Sex Crimes Squad are there to greet him. The 67-year-old is handcuffed and taken to the local police station, where he's presented with one of the most heinous child sex charge sheets the state has ever recorded. His victims include not only Amy and Sam but Patrick's two daughters, and the niece of another stepdaughter.

The indictment numbers 77 counts, with offences occurring from at least 2003 through to 2015. It details the grooming, manipulation and blackmail of six alleged child victims in his home, his car, his caravan and a South Coast motel. And it canvasses the spectrum of sexual abuse inflicted on them, the supply of drugs and alcohol, and the false promises of modelling contracts to coerce them into sitting for nude photos.

Prior is sitting in a small conference room at the back of Bankstown police station, where she is based. She's tough but approachable, with an air that suggests she's a good listener. She calls Thomas a master manipulator, grooming the children's families as much as his victims. "Laypeople would say, 'How did the parents not know?' " she says. "But in these sorts of cases, in a family environment, most people don't pick up on warning signs, and this is what [Thomas] was so good at: manipulating these families to shut them up.

"Once you finish with their story, you [ask] yourself, 'Where does the truth lie in this?' With Amy, the truth lay in everything. We were able to corroborate absolutely everything."

When she considers the numbers, Prior can hardly believe the scale of the case against Thomas, and the task she had in building it. Six alleged victims, 50 volumes of evidence, more than 80 prosecution witnesses, 10 of them from interstate. At trial, when not in the witness box she sat behind the prosecution, second-guessing her every move. "I would say to my team, 'What if I've made a mistake?' On a crime-scene warrant, if you have even a date wrong ... you lose it. If you lose your crime-scene warrant, you lose your crime scene. You lose your evidence."

Thomas pleaded not guilty to all 77 charges against him. "You've got to prove 77 offences," says Prior. "It was extremely difficult. When you've got offences carrying 10 years, 25 years, carrying life ... reaching every single proof of that beyond a reasonable doubt is huge."

The investigation was Prior's final one in almost 13 years with the NSW Child Abuse and Sex Crimes Squad; after it, she requested a transfer to another unit, and has since been promoted to detective sergeant. "I've not seen a paedophile as bad as him, and I've seen some pretty awful, awful people." She pauses for a moment. "A case like this never leaves you."

On March 8, 2018, Judge Robert Sutherland of the NSW District Court finds the now 71-year-old Thomas guilty of 73 charges - involving five of the six complainants - of the 77 he faced.

The 97-page judgment takes two days to be read to the court. "The affront to the psyche of each of these children cannot be overestimated," Judge Sutherland says in his final sentencing, calling out the lack of remorse or contrition shown by the elderly accused, who continued to blame the children. "The psychosocial mutilation of young developing minds occasioned by such conducts ... can lead to very substantial, if not lifelong impairment."

It is a damning assessment of a man described by consultant forensic psychiatrist Jeremy O'Dea as "relatively physically fit and healthy" but "vague, contradictory and eluding" in his denial of the abuse, with "apparently self-serving explanations displaying limited empathy for the victims". In court, O'Dea recounted a prison interview he conducted with Thomas in early 2018, in which Thomas told him that "most of it didn't happen ... it's a payback for taking them all out of my will". He argued that "all the kids lied in court ... they were coached by the same person ... I can only guess who."

Thomas was not suffering from a major psychiatric illness, O'Dea concluded, nor did he report awareness of his own paedophilic urges or fantasies. O'Dea recognised instead "coercive and deceptive" behaviour, his "apparent lack of insight and acknowledgment" a sign of antisocial and psychopathic traits potentially describing a personality disorder. Thomas's conduct, he said, fitted at least the medical diagnostic criteria for a paedophilic disorder, with "a specific and strong, if not exclusive, paedophilic component to his sexuality".

There is a powerful energy in Sydney's Darlinghurst Courthouse in June 2018 as the man Amy and Sam once called Opa emerges, at the top of a concealed staircase, for his sentencing hearing.

"For the duration of my life I have felt the seismic aftermath of sexual abuse," Sam, now 16, begins, reading his victim impact statement, just metres and a layer of glass from the man who abused him for so long. "The people in my life that I thought I could trust became people that terrified me. Someone who I once considered a grandparent, a carer, somebody I loved and trusted was now the one person who made me wish I wasn't me."

Spurred on by her brother's confidence, Amy, now 18, also takes the stand. "The abuse I have suffered should never have happened to me," she declares, her voice quivering at times. "I was only a child and my childhood was taken away from me ... I should have been nurtured and loved and protected by my grandparents, but instead I was manipulated by an evil, sick, twisted, selfish, weak old man. Thomas, you are nothing but a coward and you deserve to spend every waking moment knowing that I am stronger than you ... I will rise above the trauma and continue with my life and all the good that is in it."

Later that year, on August 28, Thomas is sentenced to 32 years in prison, with a non-parole period of 24 years. With time served, he'll be eligible for release in 2039, aged 91.

On a warm Sunday afternoon, Nick and Maria are sitting side-by-side in the living room of their home on the South Coast. The peace and quiet of their surroundings - barely a car drives past - stand in stark contrast to the tales of horror they're relaying. They still find it hard to comprehend how their very normal, middle-class family ended up here. How a man they had known for so long was able to so systematically manipulate and brainwash their children. How his wife Sarah - Maria's mother - sided with Thomas over her own daughter, believing his version of events.

"The deceit was very hard to cope with," Maria says. "I felt like, as a parent, I had failed to protect my kids. I have a lot of regrets of not reporting stuff over the years, but that's what I want people out there to know."

Nick never speaks directly about the acts of abuse; he has a tough exterior, but it seems as if openly detailing it might break him. "He's beyond evil," he says, with a low voice and a piercing glare. "Your grandparents should be there to spoil you a little bit ... I just thought he was being over-the-top with the presents, trying to undermine us. I think he had it planned right from the beginning." Raised in a tight Italian-Australian brood, Nick is no stranger to overbearing relatives. In hindsight he says it's difficult to ignore how hard Thomas worked at being close to the children. "He was showing a very unhealthy interest ... he wanted to be there 24/7. And I was a bit naive, thinking the kids would tell me if anything untoward was happening."

Maria now understands just how taboo a topic is intra-familial sexual abuse, despite it being considered the most common type of child sexual abuse in Australia. "Even if I talk about it to people, they don't want to hear it," she says. "But by not talking about it, we are empowering paedophiles. I believe educating children from a very young age about consent and body safety will empower them to keep themselves safe and identify if they are being groomed." Maria has begun working with the University of Wollongong's SAYER Foundation, a social enterprise which offers support for victims of domestic and family violence.

Nick and Maria can't change the path of their own family's tragedy, but they hope that in telling their story, they can help others. "Everyone should be reporting," Maria says. "People see something, they think, 'What if I'm wrong?' and they say nothing, but if more people had reported, then our kids might not have suffered as long as they did or as much as they did. I want this story to go out so other kids know it's safe to come out ... that families see how this can happen. It needs to send a message to the community: that we aren't going to accept this type of crime."

Asked about last October's apology to institutional child sex-abuse victims by PM Scott Morrison, Nick and Maria say, "The kids feel like it does not include them ... like their kind of abuse is not recognised."

After their journey to hell and back the family moved house, in an attempt to make a fresh start. Set on a corner block, their new four-bedroom home, nestled within a tidy, modern housing estate just minutes from the beach, has a holiday vibe. "It's all new, [including] the furniture," says Maria. "We wanted somewhere he had never been."

For Amy and Sam, Thomas's sentencing marked the first day of the rest of their lives. Both have finished school, are maintaining counselling and are working, with Sam recently commencing study at TAFE. It's a transition that Nick says has been "huge for them".

"The change has been dramatic," adds Nick, with a distinctive sound of hope in his voice.

*Some names in this story have been changed.

This story was first published in the Good Weekend magazine.