The jury in the murder trial of a New South Wales prison officer who shot a shackled Wiradjuri man dead has been discharged after failing to reach a verdict.

Dwayne Johnstone, 43, had his hands and ankles in cuffs when 'Officer A' shot him dead outside Lismore Base Hospital on 15 March 2019.

At the beginning of the trial Crown prosecutor Ken McKay SC said Mr Johnstone was "posing no risk to any person at the time he was shot".

The trial in the New South Wales Supreme Court lasted three weeks, followed by almost one week of jury deliberations which failed to reach a conclusion.

Mr Johnstone was being escorted out of the hospital when he shoved a prison officer and ran across the road.

The other officer drew his revolver and shouted a warning before firing three times, hitting Mr Johnstone in the back with the third and fatal shot 11 seconds after the escape attempt began.

The defence argued Officer A had a duty to prevent an escape.

Phillip Strickland SC said the officer did not act out of malice or anger, and shouted a warning before firing two warning shots.

"You must give a warning to the person escaping to give them a chance to stop. Not once but several times," he said.

In an early police interview, the officer admitted aiming at Mr Johnstone's centre of mass, which Mr McKay said represented one element of murder.

Mr Johnstone had been arrested on 14 March 2019 and charged with assault and possession of a taser.

At least 518 Indigenous people have died in custody since the end of the Royal Commission into Aboriginal Deaths in Custody in 1991.