The teenage victim met her alleged attacker, Rashadul Islam in a random encounter after leaving her residential out-of-home care facility last month.

She was then reported missing by the facility, located in Sydney's west, but was not seen again until the early hours of Thursday morning.

About 1.30am on Thursday, police saw her allegedly being chased down Balmoral Street, Blacktown, by Mr Islam.

Officers allegedly saw the 29-year-old grab the girl and try to drag her away before they intervened and took him to Blacktown police station.

The girl told officers Mr Islam had allegedly been holding her against her will and sexually assaulting her for four weeks at a house in Blacktown.

"Inquiries also revealed the girl had been reported missing from a residential out-of-home care facility earlier this month," police said in a statement.

"She was taken to The Children's Hospital at Westmead for medical assessment and has since been released."

The Department of Family and Community Services have been told of the incident, police said.

Officers raided Mr Islam's house and seized a number of items.

He was charged with aggravated sexual assault, common assault. The matter was briefly heard at Blacktown Local Court on Friday where Mr Islam did not apply for bail and it was formally refused.

It is understood he was in Australia on a current visa.

"Police also notified the Department of Immigration and Border Protection, regarding the man's arrest," police said.

Inquiries into the incident are continuing.

Mr Islam returns to court on Tuesday.

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