The man, who cannot be named for legal reasons, had been in a relationship with the children's mother for several years and had a child together before he began molesting her two oldest children.

Court documents said the man repeatedly abused both siblings over a period of years, often when he was in close proximity to their mother.

The teen, now 16, recounted an incident to police from mid-2018 where he was molested while the family watched a movie in the lounge room.

He said he and his step-father were sitting on one couch, and his mother and sister on another, when the man reached under a blanket covering both of them and began to fondle his step-son's penis.

He said the assault continued for five minutes and left him scared and afraid of what to do so he didn't tell anyone.

Then, in January this year, the victim said he and his step-father were sitting in the back of a car with his mother and a female friend in the front seats when the man again fondled his penis before undoing his own fly, and taking out his penis, which he covered with his shirt.

The man dismissed the teen's hushed concerns, saying "shhh, be quiet", then grabbed the victim's hand and put it on his penis, forcing him to stroke it.

The victim told police he was accosted again in June this year, after the family had returned from a night out.

The teen said he was in bed in his pyjamas, watching videos on his phone when he heard his mother go downstairs and soon after, his step-father entered his bedroom.

After questioning the teen about the messy state of his bedroom, he knelt next to the bed and started performing fellatio on him.

The boy heard his mother yell out "what are you doing up there?"

His step-father responded "telling [the teen] how great he played today", referring to a sporting competition the victim had been involved in that day. He then continued to perform oral sex on the teen.

However, the victim's mother unexpectedly entered the room a few moments later and turned the light on, at which time she saw her son with his penis out and her husband on his knees, with his head close to the victim's groin area.

She said "what the fuck are you doing"

The man got up, moved back from the bed and said "nothing", prompting his wife to reply "his penis was out."

The man said "we were watching basketball" then walked out of the victim's bedroom.

The mother spoke to her son, asking what had happened.

He disclosed the abuse to her then locked himself in the bathroom and refused to come out until his step-father left the house.

The mother took her son and daughter to the police station the next morning to report the abuse.

Detectives searched the family's home later that day and seized linen from the victim's bed however the man had already washed his step-son's pyjamas.

He was arrested and taken to Lake Illawarra Police Station where he was charged with a host of child abuse offences.

The man denied abusing his step-son when interviewed by detectives.

However, just days later, the man's step-daughter came forward and disclosed she too had been abused by the man.

She recalled the first incident, sometime in either 2015 or 2016, when she was aged between 9 and 11, her step-father entered her bedroom and began rubbing her vagina while she was in bed.

She said she was abused on several more occasions over the years, saying the last incident occurred in April this year when he again touched her on the vagina.

The girl told police she ran from her bedroom and slept in her mother's bed that night and her step-father fell asleep on the lounge.

She said she didn't tell her mother about the abuse because she didn't want her parent's relationship to be affected.

The man has been in custody since his arrest.

He will be sentenced at a later date.