A teenage girl from West Africa has been held in a Sydney home against her will and used as a sex slave for weeks before managing to escape.

Police are searching for a woman hailed as a Good Samaritan who found the 17-year-old after she escaped in the early hours from a Sydney home on April 27 and dropped her at an innerwest Asylum Seeker Centre.

The teen from Guinea says she arrived in Sydney in early April and was driven to a home believed to be in the city's metropolitan area - where she was kept in a room and sexually assaulted by a number of men.

The AFP's Human Trafficking Unit has been brought in to investigate along with the NSW Police's State Crime Command's Sex Crimes Squad.

The girl says in January she met a man in her home country who offered to bring her to Australia as his cleaner and in April they travelled together via Paris to Sydney.

It's believed the woman who rescued the teen is called Nicole and she drives a small red car, police said today.