NSW Police are investigating an incident at Sydney's Villawood detention centre following allegations that a male detainee was gang-raped by three other detainees.

Police have confirmed detectives from Bankstown Local Area Command are investigating an incident reported to have occurred on July 4. The incident, if proven, raises serious questions about the duty of care provided to those the federal government keeps locked behind wire in immigration detention.

Fairfax Media has been told of an alleged violent attack last month in which a male detainee was raped by three other male detainees in an area of the detention centre not covered by security cameras.

"It was in a blind spot … where the cameras don't pick anything up," a source who claimed to have knowledge of the incident said.