Australian Federal Police assistant commissioner Colin Winchester was shot twice in the head at close range as he parked in the driveway next to his Canberra home at about 9.15pm on January 10, 1989.

David Eastman has pleaded not guilty to murder.

Responding to the prosecution case in the ACT Supreme Court on Wednesday, defence barrister George Georgiou SC said Eastman had not been driven to kill because of an assault charge he thought would bar his return to the public service.

"The alleged motive is strongly disputed," he said.

"By the time of the death, Mr Eastman had been deemed fit to return to work."

The prosecution alleges the 72-year-old's motive was "murderous hatred", which came to be directed at Mr Winchester after he was unable to help with dropping the assault charges.

Mr Georgiou said Eastman had gone to the police saying he had been assaulted, and ended up being charged instead of his neighbour.

Eastman is alleged to have written a letter to Irene Fink saying he wanted to kill his neighbour, his friends, and the "bastard" police.

"There's no issue he had a strong sense of grievance," Mr Georgiou said.

"They were expressions of frustration. Often in the heat of the moment.

"It is disputed, it is an issue, that they reflected a murderous intent."

The prosecution told the jury earlier in the trial that listening devices placed in Eastman's flat revealed him whispering to himself "he was the first man, the first man I ever killed."

Mr Georgiou said they were indiscernible whisperings and murmurings, and they needed to be heard in context.

"Don't pluck out the individual words. Context can place individual words in a completely different light," he told the jury.

The murder weapon was never found but police identified it as a Ruger 10/22 .22 calibre semi-automatic rifle sold by a Queanbeyan man on January 1, 1989.

While the prosecution said a witness would testify to seeing Eastman at the home of the gun seller, Mr Georgiou said this would be disputed and denied.

There was a "reasonable possibility" that Mr Winchester was killed by someone tied to the mafia, he said.

Mr Georgiou told the jury it was up to the prosecution to prove the charge of murder beyond a reasonable doubt.

Australian Associated Press