Police have no idea why a man, who was shot dead inside a police station in Sydney's west, walked into the premises and allegedly started threatening officers.

The man, believed to be aged in his 40s, is yet to be identified, but police say yesterday's incident at Quakers Hill police station is not believed to be terrorism-related.

A sergeant with 24 years' experience fired the fatal shot after an altercation that lasted just seconds at 10.40am local time, Assistant Commissioner Denis Clifford said outside the station.

A number of officers were in the police station and a 12-year old boy was also in the foyer when the man entered the building.


"There was a shot fired by the sergeant. It struck the man. He was rendered assistance but passed away a short time later," Clifford said.

He said police "have no idea of the motive" of why the man entered the station. "I can say that a large knife has been located at the scene and obviously that will be part of the investigation and the crime scene unit are on the scene now to try and piece together what occurred.

"I've spoken to a number of officers in there and the officer who fired the shot is very distressed, as you could understand," Clifford said. "But I can say that no police have been injured, which is great."

Police officers have been identified as targets of terror attacks inspired by Isis (Islamic State).

A 15-year-old boy was shot dead by police soon after he fatally shot a civilian police employee outside the state police headquarters in Parramatta last October. An 18-year-old man was shot dead after he stabbed two policemen outside a police station in Melbourne in September 2014. Both attacks have been blamed on Isis followers.