A fake passport may have been used to bring a West African teenager to Sydney where she was allegedly held in captivity and used as sex slave, police say.

As part of the joint investigation, the Australian Federal Police (AFP) and the NSW police have appealed to the public for help as they try to track down a man who allegedly flew the 17-year old girl from Guinea to Sydney in early April, on the promise she would work as a cleaner.

"Obviously how she got here, the passport she travelled under and who she travelled with are critical lines of inquiry," AFP Commander Chris Sheehan said.

The investigation had already come up against a number of barriers which meant crucial details, including the passport information, had yet to be confirmed, police said.

"This has been a very, very difficult investigation, there are cultural issues, language issues to deal with," Acting Superintendent Mick Haddow said.

"The statement has taken days and days and days to complete."

Police understand the girl was held inside a room of the house where she was brought to and then sexually assaulted by several men.

Good Samaritan asked to come forward

Acting Superintendent Haddow said the girl escaped in the early hours of the morning on April 27 and was picked up by a woman thought to be named Nicole who drove her to the Asylum Seeker Centre in Newtown.

"The girl has flagged down the vehicle," he said.

"She [Nicole] is a really important witness."

Police have not yet been able to find the apartment where the girl was held.

"It's really vague descriptions about units, we don't know what suburbs we are talking about at this stage," Superintendent Haddow said.

"We believe it's possibly close to the Newtown area.

"It could be kilometres and kilometres away from that area."

The girl was then taken to Royal Prince Alfred Hospital before being placed into the care of Family and Community Services.

The AFP Human Trafficking Unit is continuing its inquiries to determine when the girl arrived in Australia, and the identity of the man she was travelling with.

The New South Wales State Crime Command are appealing for the woman who picked her up, to come forward.