The man, who cannot be named for legal reasons, admitted repeatedly molesting the girl - at times while simultaneously performing sexual acts on himself - over a two year period while she lived at his house. She was aged between six and eight at the time of the offending, but is now in her teens.

The man, who remains behind bars, faced Wollongong Local Court this week where he pleaded guilty to 18 charges of aggravated indecent assault.

Court documents said the girl's mother considered herself incapable of caring for her children and arranged for the girl and her brother to live with the couple, who she knew through a friend.

The man began molesting the girl within a week of her arrival, taking advantage of her vulnerable state and his wife's absence from the house due to her work schedule.

At the end of each incident, the man told the girl "I'm sorry, I will never do it again, I didn't mean to do any of that. Promise me you won't tell anyone".

However, she told police the abuse continued for the next two years.

She subsequently returned to live with her mother however still visited the family on weekends. She said the abuse happened "so regularly" it was impossible to name exact times and dates.

"The victim told police that at the time these offences were occurring she didn't understand what the accused was doing as she was so young," detectives wrote in court documents.

"It was not until she became older and was in high school where she took a health lesson and was taught about healthy and unhealthy relationships that she realised the reality of it."

The girl subsequently reported the allegations to police and the man was arrested.

It was not revealed in court whether the mother knew of the man's prior conviction when she left the children with the family.

He will be sentenced next month.