A NSW court has heard that a man accused of sexually assaulting a 13-year-old girl in Lake Macquarie was already on bail for numerous similar offences.

Police allege John Blair Burns, 28, used social media to groom the teenage girl for a year before allegedly sexually assaulting her in a home near Lake Macquarie, NSW.

Police stopped the 28-year-old while he was driving a car early Sunday morning.

They said the 13-year-old girl and a 16-year-old boy — who are not related — were inside the car.

Police interviewed the girl, who told them she had been sexually assaulted by the 28-year-old man.

Both the girl and the boy were taken into police care.

The man was arrested and then taken to Toronto police station, where he was refused bail.

Police charged the man with several sex offences, including aggravated sexual assault with a victim younger than 16.

Mr Burns was on bail for several other alleged offences.

He did not appear in Toronto Local court today where his legal representative did not apply for bail and it was formally refused

Mr Burns's legal representative said his client admitted to breaching his bail curfew, but denied the fresh allegations against him.

A police spokeswoman said the Department of Family and Community Services had been notified.

Both children are back with their families.