Police are looking for five men over the abduction and sexual assault of a young woman in Sydney's northwest.

The 18-year-old woman left a house party in Baulkham Hill about 1am (AEDT) on Sunday and was walking alone when a green car stopped next to her near St Michael's Place.

One of five men in the car asked for directions to a shopping centre, before another grabbed her from behind and forced her into the car, police say.

The woman was driven to an unknown location and sexually assaulted by a number of the men, before being dropped off in Castle Hill.

The victim phoned police when she got home and was taken to Westmead Hospital.

Local police and the State Crime Command's Sex Crimes Squad are investigating.

They have been told the car was spotted outside the party on Merindah Street earlier in the night.

Anyone with information is urged to contact Crime Stoppers on 1800 333 000.