A Sydney man has been refused bail after being charged over a live-in sexual relationship with a 13-year-old girl.

The pair were living together in Sydney's west.

Police sources say the 26-year-old man and the girl were "married" in an Islamic religious ceremony in the Hunter region last month.

The marriage has no legal standing in Australia because of the girl's age.

The man, who cannot be named for legal reasons, allegedly met the girl in the Hunter region last year and began an ongoing sexual relationship with her.

They then moved in together in Sydney.

A nearby high school contacted police recently when the man, a Lebanese national, tried to enrol the girl.

Detectives from the state's Child Abuse Squad arrested the man yesterday.

He was then charged with 25 separate counts of having sexual intercourse with a child aged between 10 and 14 years.

The man needed an interpreter when he faced Burwood Local Court today, where bail was refused.

He will apply for bail again when he reappears next week.

The Child Abuse Squad says its investigations into the case are continuing.

Police have questioned the girl's parents and believe they had a hand in arranging the marriage.

Detective Inspector Peter Yeomans says it is a disturbing situation.

"It's really really unusual," he said.

"We do have sexual assaults of minors... but this type of situation where it is almost sanctioned by a family, if that ends up being the case, is a very very unusual set of circumstances to happen within NSW."

But the NSW Minister for Family and Community services, Pru Goward, says it is not an isolated case.

"There are actually a significant number of unlawful, unregistered marriages to underage girls in NSW, particularly in south-west Sydney, western Sydney and the Blue Mountains," she said.