Five people are facing charges of aggravated sexual assault after police investigating graffiti offences in western Sydney stumbled upon footage of a gang rape on a GoPro camera.

Key points: Girl, 16, allegedly sexually assaulted by up to eight men at a party

Girl, 16, allegedly sexually assaulted by up to eight men at a party Footage of the attack found on a camera seized in a graffiti investigation, police say

Footage of the attack found on a camera seized in a graffiti investigation, police say Four men and one boy charged

Four men and a 17-year-old boy have been arrested and charged over the alleged assaults, but police said up to eight men were in the room when the victim, a 16-year-old girl, was attacked.

The assaults allegedly took place at a party at the western Sydney suburb of St Clair in May.

Police said the alleged victim was either unconscious or semi-conscious at the time and the attack was not reported, but officers have since contacted and interviewed her.

"In May, police seized a GoPro camera as part of a police operation targeting graffiti offences and found a video depicting multiple sexual assaults of a teenage girl by a number of male offenders," a NSW Police statement said.

"Police will allege up to eight males were in the room at the time of the assaults, with a number of them sexually assaulting the girl."

Detective Chief Inspector Peter Yeomans, from the Child Abuse Squad, said the alleged victim was undergoing counselling.

"It was quite obvious to investigators when viewing this footage that the child was either unconscious or semi-conscious during these assaults," he said.

Those charged include a 17-year-old boy arrested at Willmot; two men, aged 24 and 23, arrested at Penrith and another 24-year-old man arrested at Mt Druitt.

They were all charged with multiple sexual offences, including aggravated sexual assault in company.

A 25-year-old man, arrested at St Marys, was charged with concealing a serious indictable offence.

Two of the men have had their cases adjourned without applying for bail.

Kurt Stevenson, 24, appeared on screen at Penrith Local Court, while 23-year-old Andrew Waters was represented by his lawyer and did not appear on screen or in the dock.

A police prosecutor told the court Tristan Carlyle-Watson, 25, was present during the attack and encouraged others.

It was also alleged he claimed to know nothing about it when contacted by the victim the next day.

A magistrate refused bail, saying there was an unacceptable risk of further offences, including interfering with witnesses.

Others charged are due to face court at Mt Druitt today.