Alleged gang rape disgusting: NSW police

The gang rape of a 16-year-old girl in a Western Sydney car park stairwell has disgusted police.

The girl was in the Liverpool library forecourt on Saturday night after using the council's free wi-fi internet when six men approached her and invited her to meet a mutual friend.

She was then allegedly sexually assaulted by two of the men in the stairwell of a nearby car park while the other men failed to stop the attack, police say.

The teenage girl is traumatised says Superintendent Peter Gillam, but she is helping police with their inquires.

"(I'm) obviously very disgusted," he told reporters in Liverpool on Tuesday.

"It's something that shouldn't happen in Liverpool or anywhere."

After the assault, the girl returned to the library forecourt and told a bystander, who called the police.

The men, described as African in appearance and aged in their 20s, were last seen walking toward a Macquarie Street shopping centre.

Supt Gillam stressed that the attack appeared to be a one-off, random incident and had nothing to do with any race-related issue.

Detectives are calling for anyone with information to come forward.

The attack follows two similar assaults in Sydney's west this year.

In February, a girl was allegedly the victim of a pack rape in a Doonside park.

Four teenagers were charged over the alleged attack.

Two months later, two teenage girls were coaxed off a Sydney train and raped in a Fairfield park.