A weapons expert has defended the actions of a police officer who shot dead a man at Armidale in north-west New South Wales almost four years ago.

Elijah Holcombe, 24, was mentally ill and brandishing a knife when he was shot dead by Senior Constable Andrew Rich in June 2009.

Police had been called by Mr Holcombe's father who reported his son had stolen his car.

At the time, he was suffering from delusions and paranoia - including being afraid of police - after having a mental breakdown.

Police officers arrived at the scene, where Mr Holcombe was armed with a bread knife, with the intention of taking him to hospital.

He was shot as he was chased into a laneway.

The coronial inquest into the shooting has been re-opened after a three-year adjournment.

The inquest began in 2010 but was later stopped because a coroner believed criminal charges could be laid, however the Director of Public Prosecutions last year announced that would not be the case.

Today the inquest at Glebe Coroner's court heard from police weapons and tactics expert Senior Sergeant Peter Davis.

He told the court that Senior Constable Rich did no wrong when he chased Mr Holcombe through a cafe where the man armed himself.

He says holding a knife is not a reason to discharge a gun, but the victim ran aggressively towards Senior Constable Rich with his knife out as a threat to him.

Other methods

Senior Constable Rich did not have his capsicum spray on him at the time, but even if he did, Senior Sergeant Davis told the court it does not have immediate effect and the victim was too far away.

He says a Taser may have been of assistance but they were not introduced into the Police Force until later that year.

When giving evidence, Senior Constable Rich said he asked Mr Holcombe to drop the knife.

Instead, Mr Holcombe said various things including "shoot me", and started running in Senior Constable Rich's direction.

Senior Constable Rich then shot him.

Senior Constable Gregory Dufty, Senior Constable Rich's partner at the time of the shooting, and who is now retired, said he had been receiving treatment for two years for post-traumatic stress disorder after the incident.