The bullet that hit his heart was the third fired by the corrections officer, who has pleaded not guilty to murder.

The man cannot be legally identified, besides under the pseudonym Officer A.

His barrister Philip Strickland SC told a NSW Supreme Court jury on Tuesday the man identified himself as a corrections officer and shouted "stop or I'll shoot" before firing three times.

“When he fired the third shot, he believed he was doing what was necessary to prevent the escape, that was his job,” he said.

Mr Strickland told the jury the officer fired a warning shot which hit a railing and a second shot which went through the window of a closed medical clinic before the third shot hit Mr Johnstone.

Prosecutors disputed that the first was a warning shot during an opening address on Monday.

Mr Johnstone was taken back to the hospital after being shot but died about two hours later.

Prosecutors allege the shooting occurred without lawful excuse.

Officer A told police Mr Johnstone was running quickly despite having shackles around his hands and ankles.

The officer did not believe either he or another corrections staff member accompanying him could catch up.

Mr Strickland said the jury would have to consider whether the officer acted necessarily to prevent an escape.

“When you consider the issue of reasonable grounds, look at it through the eyes and perspective of the accused,” he said.

CCTV footage that captured the 11-second period before Mr Johnstone was shot came from about 95m away and showed a different perspective than the officer had at the time, Mr Strickland said.

Experts would tell the trial how perspectives could be exaggerated and distances could appear inaccurate on footage due to the position of the camera, he added.

The trial continues and is expected to last about four weeks.