Police sources say a 15-year-old Iraqi-Kurdish boy shot dead a police employee in west Sydney yesterday afternoon.
Father-of-two Curtis Cheng was shot outside the Parramatta Police Headquarters around 4:30pm.
Police officers and security guards returned fire from the building lobby, 9 News reports.
Investigators are scouring CCTV after the shooting death.
New South Wales Police Commissioner Andrew Scipione says they won't leave any stone unturned in the investigation.
Mr Scipione says the boy worked alone in an act of terrorism. He can also confirm the boy was born in Iran.
It's believed members of the teenager's immediate family have recently returned to the Middle East.
The New South Wales Police building on Charles St is where administration and support teams are based, alongside with the State Crime Command.
Mr Cheng was a 17-year veteran of the New South Wales Police finance department, according to 9 News.
RadioLIVE / 3 News