New South Wales police have confirmed reports that the person who shot a police worker in Sydney's west yesterday was a teenager.

The staff member, Curtis Cheng, who worked in the accounts department and had been with the police for 17 years, was shot at close range as he left work at the State Crime Command in Parramatta.

His killer, a 15-year-old radicalised youth born in Iran, was shot dead as officers returned fire.

Commissioner Andrew Scipione said the killing was targeted.

He said there's no information to suggest anyone else was involved.

He said the killing was an act of terrorism.