'No reasonable opportunity' to stop 15yo from killing Sydney police worker

Police accountant Curtis Cheng, 58, was shot outside NSW Police headquarters in Sydney's west. (AAP: David Moir)

An inquest into the shooting death of a police accountant has found law enforcement could not have intervened before it happened.

Curtis Cheng, 58, was shot by Farhad Jabar Khalil Mohammad, 15, outside a police station in Sydney's west.

Deputy State Coroner Derek Lee said a police officer used appropriate force when he shot dead the teenager.

An inquest into the shooting death of Sydney police accountant Curtis Cheng has found there was "no reasonable opportunity" for law enforcement officials to intervene prior to the incident.

The 58-year-old police civilian employee was shot and killed by 15-year-old Farhad Jabar Khalil Mohammad outside police headquarters in Parramatta on October 2, 2015.

The teenage gunman, known to NSW Joint Counter Terrorism Team (NSW JCTT) investigators, was then shot by a special constable at the scene in Sydney's west, after being alerted by the sounds of gunshots directed towards the building.

Deputy State Coroner Derek Lee, in delivering his findings on Friday, said the fatal attack on Mr Cheng had been the result of violent Islamic extremism.

"The extremist ideology directed towards police made the events all the more shocking," Magistrate Lee said.

Curtis Cheng (left) pictured with his family, was shot dead more than 10 years ago. (Supplied: NSW Police)

The deputy state coroner said while there had been some surveillance of a group surrounding the radicalised teen, he had not been individually identified as at risk of violent extremist acts by NSW JCTT investigators.

"While Operation Peckham investigators had correctly identified persons of interest were suspected of planning a specific terrorist act at a specific location, the intelligence gathered did not allow for the details of the participants to be identified," he said.

He said the teen was not the subject of any specific surveillance.

Magistrate Lee said it was only after the incident that investigators were able to form an accurate picture.

He said it would not have been possible to prevent the shooting of Mr Cheng.

"This limited time frame and the absence of any information and intelligence to suggest Farhad Mohammad was a participant meant that there was no reasonable opportunity for law enforcement authorities to intervene to prevent the shooting of Mr Cheng."

A plaque was commissioned to honour the former NSW Police employee. (ABC News: Abubakr Sajid)

Magistrate Lee also said his findings showed appropriate force had been used by NSW