The man, who cannot be named for legal reasons, began abusing his stepson in 2001 when the boy was just six years old, forcing him to engage in sexual acts "about once a month" over the space of 10 years.

The man later came to treat his then 16-year-old stepdaughter like a girlfriend, admitting the two had spoken of running away together.

"We started having an intimate relationship since she was 16 ... basically we were together as like boyfriend and girlfriend," the man told police when he was interviewed over the allegations in October 2011.

"We'd kiss and cuddle and, you know, go places together ... have sex.

"[We'd] have sex from once to four times a week."

The man told police the girl wrote him letters detailing how she wanted them to move out together and start a family, and that the two had discussed such a scenario.

He also told her at one stage that he loved her "more than her mother", whom he had begun dating in 1999, before moving in with her and the children in 2000.

The ongoing abuse towards the boy finally came to light when the now 18-year-old spoke to his mother in March 2011, after having threatened suicide a month earlier.

At the time he refused to go to police with the information, despite having the support of his mother.

In April 2011, he disclosed the abuse to his father and stepmother, who took him to Wollongong police station where he reported the matter.

His sister, now aged 21, told a neighbour about the sexual relationship between herself and the man in September 2011. She also subsequently provided a statement to police.

As a result of the allegations, police interviewed the man's natural daughter (who was also the children's stepsister) in October 2011.

She revealed her father had also sexually assaulted her during 2011, when she was aged 10 or 11.

The man had originally pleaded not guilty to 11 charges stemming from the abuse allegations and was due to stand trial in Wollongong District Court yesterday, but changed his pleas to guilty before the trial began.