In the early hours of Sunday, October 28, 2012, my 95-year-old father killed my 85-year-old mother. For the previous 18 months, there'd been a growing and terrifying possibility that he might feel compelled to do this, but my mind kept steering away from such a thought. I couldn't take her away from him – and nor would I. They'd been married for 65 years. But I knew the possibility was there and I just kept hoping that she would die before it happened. He was hoping that, too. 

My mother, Olive Chapman, was only 16 when she and my father first met in February, 1944. Fred Bagnall, 26 years old and an army reservist, was woodcutting at Fletcherview, Granny Chapman's cattle property near Charters Towers in North Queensland, and had to go up to the main house for some pepper because his car's radiator had sprung a leak (using pepper to clog a hole was a common temporary fix). That same day he'd been pig shooting with his brother and had seen a dozen piebald piglets scatter into the scrub behind their camp. Just before he left to go up to the house, Dad found one of the piglets hiding under his swag. He named her Peggy and made a sling to carry her.

Mum was feeding the chooks at Fletcherview when she saw him coming down the red-dirt road. Every year after that, at Christmas when my parents made a toast, Mum would always say, "I saw Dad coming down the road with his arm in what I thought was a sling and I said to myself, 'He's the one for me.' " 

Mum wasn't allowed to date until she was 18, but Dad saw her regularly in the company of a chaperone. The first time Fred kissed her, she slapped him and he apologised – and so did she. They always re-enacted The Slap during their Christmas toasts. Dad would get up and make the toast and then Mum would hang her arms around his neck and say how gorgeous he was – and he'd get all embarrassed and say, "Sit down, you're tipsy." He was very embarrassed by public displays of affection.  

In north Queensland's big floods of 1946 – the worst on record – Mum and her family had to climb out onto the roof of Fletcherview after the water reached the second storey.  They stayed there for 10 hours. Mum had all Dad's love letters wrapped in her Driza-bone. He was stranded somewhere north of Ingham. Once he'd got back to Charters Towers, he swam two flooded creeks to find her. They got married 18 months later on October 1, 1947. My sister Robyn was born in 1950. Then came Betty in 1953. Then, five years later, me. 

Mum thought Dad handsome; Dad loved her because she was a sweet, innocent little girl. He had strong ideas about women and how they were supposed to be. He would have been about 45 when I was 14 or 15 and started wearing miniskirts and hot pants. Dad had no time for any of that: he thought society was going to wrack and ruin. 

In early 1930, when Dad was a boy in Pentland, the small town in north-western Queensland where he grew up, and everyone was on the post-office verandah dancing and cheering to celebrate Don Bradman's world-record score of 452 in an interstate match, Fred danced a jig and said, "Well, I'll be buggered." His dad chased him with bridle reins and he had to spend the night sleeping with the goat because "no kid of George Bagnall's was gonna be reared swearing".  

Dad was, unquestionably, the head of the family. He was the strong, silent type, while Mum was the good little wife – with a difference: she was also an astute businesswoman. She made a wonderful success of the Tropical Café, which she started in 1964 in the main street of Charters Towers, and she did it through sheer hard work. After Dad had a kidney removed in the early 1960s and started suffering from migraines, she became the breadwinner. 

They both had a strong work ethic, though. When Dad was just nine years old, he saw an ad at the post office for a salesman. The company was Craven Burleigh & Co and among other things it sold hair-preparation treatments for men. He filled out the form and put nine as his age with a squiggle in front of it, which could have been a 1 – and got the job. He used a billy cart to sell all the goods they sent him. The best-selling thing by far was Bay Rum Hair Oil. If you sold a certain amount, you got to choose a gift. Dad chose a gramophone because he loved to listen to music. So at 10 years old, he got his first gramophone. The first record he played on it was a 1936 recording of Red River Valley by Gene Autrey. 

Dad worked on the railways as a foreman shunter when I was growing up. And when he wasn't at the railways, he worked at the café with Mum. Later, he wrote a book, North Queensland Cavalcade: A History of Charters Towers and Other Stories, which was published in 1979.

THE FIRST TIME I HEARD the word "euthanasia", I was about 13. The kids at school were talking about it. I genuinely thought they were talking about "youth in Asia". I thought, "Who gives a frig about youth in Asia?" I can't remember euthanasia ever being discussed when my sisters and I still lived at home.

Robyn dying at 54 in 2004 of leiomyosarcoma was the beginning of the end for my parents. Robyn wasn't just my big sister: she was the shining light in my life. She and her husband, Greg, lived in Brisbane, but had a holiday home in Coolum, where Robyn had chosen to have her palliative care. She'd been told she had six months to live. My parents were then 87 and 77 and Robyn honestly thought that she'd outlive them. "Don't tell them," she instructed Betty and me firmly. She lived for another 18 months. When it became clear that she was nearing the end, we couldn't keep the truth from them anymore. Dad was devastated. All the way to Coolum to see her for the last time, he kept saying, "Why didn't you tell me?" He asked me that question for years afterwards. I felt he never forgave me.

The stress of losing her daughter sent Mum, who hadn't been well for a while, on a steep downward spiral. It wasn't Dad's idea to move into Eventide Aged Care Facility in Charters Towers in 2010; it was hers. She'd been saying for ages, "It's just too much. I can't cope anymore." She was leaving the oven on, burning stuff on the stove, feeding the chooks with Omo. But after they moved into Eventide, her confusion worsened dramatically. She was lost. 

My sister Betty ran Eventide's convenience store and it was a relief to know she was keeping a close eye on them. I was living in Innisfail, four hours away, and Mum and Dad had been at Eventide for several months when I visited them for the first time in July, 2010. Mum still knew me at that stage and began to proudly show me through their unit. "This is our bedroom and we walk through the bathroom and this is Dad's office," she said. "Fred," she urged, "show Pam the other room." 

"There is no other room!" exclaimed Dad, frustrated. 

"Show Pam the other room!" she insisted. She got so angry she flew at him, shouting "I could stab you!" He caught her wrists and growled, "You're really being silly!" 

I didn't know this woman: she wasn't my mother. But it wasn't my father speaking, either. I kept remembering the last two Christmases they'd had at home before they moved here when, as usual, they'd sung You and Me, the duet made famous by country music singers Rick and Thel Carey. Dad: You and me, together life's a pleasure, and without your love where would I be? Mum: And I feel the same about you, darling … 

Dad knew Mum's dementia was only going to get worse. He'd started talking about euthanasia in the last few years before they moved to Eventide, but it was after the move that he launched his campaign proper. He never discussed helping Mum to die with me: it was all about getting permission, finding a legal way, to finish their life's journey together. He really thought he could make a difference by writing to the state premier, the prime minister, the newspapers. And I could see his point of view: "I'm 94," he'd say. "I've spent a lifetime choosing my job, my car, my hobby, my holiday. Why can't I choose the method of our last journey together and die legally with dignity?"

At one point he rang the Townsville Bulletin, saying "I want to talk to you about euthanasia", and the editor came up the same day. He must have thought he was going to get this great suicide-

pact scoop. He and Dad were talking when he looked at Mum and asked, "And you agree with this, Mrs Bagnall?" She said, "Oh no! I could never do that to my daughters." 

Dad became obsessive. He wrote letter after letter. Every argument he made would end with the plaintive "Why haven't I got the right to take my own life?" In the end, of course, he realised nobody was listening.

YEARS BEFORE MUM developed dementia, she used to say, "If I ever get like that, knock me on the head." She always knew she wanted to go with Dad – they were always going to go together – but I don't think for her it was going to be a conscious act. Mum believed in God: she didn't believe in euthanasia. She used to say, "God will take me when he's ready." But God didn't take her. Day after day, at dusk, she became frightened. Crying, she would beg Dad; "Why won't you take me home, Fred? Please take me home."

The more vulnerable Mum got, the more I loved her. She hadn't known who I was for a good while when, one day, as I was helping her on the toilet, she looked up and said, "Oh Pammy, why do you have to put up with this?" She knew me in that instant.

It's not the caring for someone that gets you: it's the simultaneous grieving for them that wears you down. Dad's biggest concern was that he was 10 years older than Mum. He said so often, "This could go on for another decade." I think he felt, in his role as husband and protector, that he had to do something while he still could. "I can't leave her here like this," he'd say. 

People with dementia fall – and often. Seeing Mum with a smashed eye socket and a black-and-blue face … I can't begin to imagine how it affected Dad. But I know one thing: he did what he thought was right. Mum had no quality of life. She'd reached the stage where she was shuffling listlessly around their unit wearing a nappy beneath a pair of Dad's underpants.

Dad could be so focused on his mission that there were times when I felt I had to say, "Dad, don't talk about Mum as if she's not here; she could be listening." He'd say, "Your mother is gone." He was already in mourning.

In 2011, Nathalie Fernbach, an ABC journalist, came to Eventide to interview Dad about euthanasia. Towards the end, Mum and I came into the room and caught Dad saying, "She's not here anymore." Mum turned to me in a rare moment of lucidity and said, "He wants to have me put down like a dog." After she died, that statement haunted me. 

ON SUNDAY, OCTOBER 28, 2012, I got the phone call I'd been expecting for 18 months. Betty said the nurses had just called her. They'd been unable to rouse Mum that morning; she appeared to have died in her sleep. 

I immediately began the journey to Charters Towers with Greg, my former brother-in-law, whom I was shortly to marry (Robyn's last words to me had been "If you and Greg should get together, that would make me very happy"). All the way, my thoughts spun around a single question: had Mum died a natural death?

When we arrived at Eventide, we were shocked by Dad's appearance. He looked so very old. His blue eyes had long since faded to grey, but now there was a faraway look in them. He looked broken. He allowed me to hug him and then patted me on the arm. He'd never been one to show emotion. We didn't talk much, but there was a sense of togetherness in the silence. 

At Mum's funeral at St Paul's Anglican Church in Charters Towers a few days later, the church was full. There was a brief moment after the service, as we were leaving, when Dad turned to face the congregation. I knew he wanted to rail at them for their hypocrisy for being there when they'd stopped visiting them after her decline. But his anger seemed to suddenly dissipate and I saw his shoulders slump. Defeated, he turned and allowed me to lead him away. 

A day or two later, when Dad and I were alone sorting through Mum's possessions, I somehow found the courage to ask him flat out: "Dad, did you do it?" He replied "No" firmly and without hesitation. I believed him because I wanted to believe him. We carried on sorting, the silence broken only by the odd, "Do you want this, Dad, or should we throw it out?"

I think his first reaction had been to keep the truth from us, not because he regretted what he'd done, but to spare us further pain. But finally he couldn't go on. The night before Greg and I were due to drive back home, he called Betty and me in for a family meeting. I just knew I was about to come face to face with a reality that I didn't want to see and for the first time in my life, I found myself raising my voice to my father. "Don't say it, Dad! Please don't say it!" But he did anyway. "Yes, I did it," he said.

He'd given her sleeping tablets. Dad had always used them; the long years of shiftwork had made him an insomniac. And I knew he had a stockpile of tablets because I'd helped him hide them, not so much because I believed he would take them, but because I believed he should have some sense of control over his life – and death. The thing is, I'd always supported him on euthanasia, but with the proviso that it was for himself, no one else. He told us that after he'd given Mum the tablets, he lay there in bed next to her and held her until he knew she'd gone. 

I wished then – and I still wish – that he hadn't told us what he'd done. Later I said to Betty, "Why did Dad tell us? We were happy thinking she'd died peacefully in her sleep." And Betty replied, "She did die peacefully in her sleep, Pam. You have to remember that. She certainly didn't have any quality of life anymore. You need to think of Dad now as he's the one who's suffering." 

There was no post-mortem. Mum was, after all, very old and frail. I'm sure people had their suspicions, though, because Dad had been such a vociferous advocate of euthanasia for so long. But if they suspected he'd had a role to play in her death, no one said anything. It felt like a tacit conspiracy.

I immediately felt that I had to put some distance between Dad and me. I realise now that I should have supported him more at the time, but I was in shock. In particular, I was haunted by visions of Mum's last moments. Had she been peaceful at the end?

A couple of weeks later, after Greg and I had returned home, Betty told me that Dad had written a letter to the Townsville Bulletin confessing what he'd done. She'd decided not to post it and I think she destroyed it instead. The fact that he'd euthanised Mum remained our secret. 

A month later, in November, I told Dad I was thinking about writing a book about Mum's life. "Why would you do that?" he said. But really, he was beyond caring. He was in a profound state of grief. He seemed to have lost his appetite and was having trouble sleeping.

For the two years that Mum and Dad were at Eventide together, I'd ring them every day, but now I found it really difficult to talk to Dad. It wasn't that I didn't want to speak to him; I just couldn't somehow. Working on the book gave me the reason I needed to check in every day: "Tell me again about the first time you met Mum," I'd say. He'd answer my questions, but never ask me anything about the book or whether I was going to reveal our family secret. 

I knew Dad was going to die: it was only a matter of time. I also knew that he would, in all likelihood, take his own life. Dad wasn't afraid of dying; he was frightened of only one thing – the pills not working. Whenever we'd discussed euthanasia in the past, he'd always said how afraid he was of waking up and finding himself lying incapacitated, only half dead, in a hospital bed. Mum had been so frail, but Dad was still very robust for a man of his age. 

ON DECEMBER 15, 2012, I rang him at 4pm. That was the time I always called to talk about the book. Greg and I lived 20 kilometres outside Tenterfield in northern NSW and were on our way home after doing the shopping. "Sorry Dad, I can't talk today," I said. "I'm on the road and the signal isn't good." 

"Okay," said Dad simply. "Goodbye." And the phone went dead. Had he hung up? That "goodbye" struck me as unusual: he never signed off this way. He always said, "Thank you for calling." I looked at the phone, wondering if I should try to call him back, but by now the signal had gone completely.

The next morning I found a voice message from Betty. "Yeah, it's Betty here, call me back." That's all she said, but I knew instantly why she was calling. The nurses had found Dad at 9am, unconscious in bed. A couple of years earlier, he'd signed a Do Not Resuscitate directive in anticipation of this very moment and Betty made sure it was honoured. He died in Charters Towers Hospital several hours later. 

Dad had told Betty the day before that he was going to take the tablets that night. "I want to be with your mother before Christmas," he'd told her. He'd added that he intended to dispose of the pill packets, and he had. Whenever we'd discussed euthanasia with him in the past, we'd always known we couldn't be involved because of the possible legal repercussions, but I think now that if I'd known what he was going to do, I'd have stayed with him that night. My sister had left him feeling deep, deep sadness at the inevitability of what he was about to do. 

Again, I'm quite sure that the Eventide staff had their suspicions but, as before, no one said anything. Watching prolonged torment can erode anyone's belief in the sanctity of life, I suppose. Despite the fact that he'd suffered an internal bleed, no post-mortem was carried out.

My father killed my mother. When I first found out what he'd done, I hated him and, loathing him as much as I loved her, I set out to write a memoir, a celebration of her life. A small life, and maybe even an insignificant life in the grander scheme of things, but a life that had been taken by my father. Had he put her out of her misery – or out of his own?

And then gradually, during the writing, my focus – and feelings – started to shift. My hatred of him had been a knee-jerk reaction. It had been their misery. The two of them were like one person. By re-creating – and reliving – my parent's memories, I rediscovered something I'd always really known. 

It had to end this way. Theirs was a love story. I can accept that now. 

The Long Goodbye by P. J. Parker (Hardie Grant Books, $30) is in shops now. 

Good Weekend, Sydney Morning Herald