Police conducting a routine graffiti operation have stumbled across a GoPro camera that was allegedly used to record the gang rape of a teenage girl in Sydney's west.

The footage allegedly depicts an unconscious or semi-conscious 16-year-old girl being raped by multiple offenders at a party in St Clair in May this year.

Police say up to eight males were in the room at the time of the offence, with a number of them sexually assaulting the teenager.

Detectives from the Child Abuse Squad conducted an investigation based on the footage. After initially pinpointing the location of the party, they then found the alleged victim and tracked down the alleged rapists.