A convicted child sex offender has been arrested and charged with molesting another vulnerable child who came into his care.

The Aboriginal girl, who was aged six and eight at the time of the abuse, came to live with the man and his wife after her mother was forced to give her up, The Illawarra Mercury reports.

Police claim the man began molesting the child within weeks of meeting her and repeatedly did so, while performing sexual acts on himself, over a two-year period that she was in his care.

It's alleged he would wait until his wife left for work before taking advantage of the young girl, and this occurred multiple times over a number of years.

The teenager later returned to live with her mother after a couple of years, however, she still visited the man and his wife on weekends.

It wasn't until she was in her teens that the girl reported the alleged assaults to police after she learned about 'healthy and unhealthy relationships' in a health class.

The girl told police officers that the abuse occurred so often that she couldn't possibly identify how many times it actually occurred.

She allegedly told officers that the type of abuse increased in its severity over the years, and eventually resulted in 'digital penetration'.

She added that at the time of the abuse she didn't realise the man was doing anything wrong because she was so young.

Police allege the man abused the child before apologising immediately afterwards, saying: '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'.

It's also believed the mother of the girl, who was a friend of the man and his partner, wasn't aware of the man's prior convictions - dating back to 1993. He has also since changed his name.

The man has since been charged with 19 child indecency offences and he will remain in custody until he faces the court next Monday.