The 17-year-old girl had been brought to Sydney in early April by a man who offered her work as a cleaner, New South Wales Police said.
She was allegedly taken to a house and sexually assaulted by "a number of men" until her escape on 27 April.
Police officers from human trafficking and sex crime squads are investigating.
The teenager told police she fled the unknown location before being picked up by a woman who drove her to a community centre.
She was taken to hospital for medical treatment.
The girl told police she had met a man in Guinea in January and he offered her employment. The pair travelled to Australia via Paris over several days in April.
Police are searching for the teenager's alleged attackers and the woman who went to her aid.