An ACT Supreme Court jury on Thursday found Mr Eastman not guilty of murdering federal police assistant commissioner Colin Winchester in 1989.

Mr Eastman, a former Treasury official, pleaded not guilty to the murder in 1993 but was sentenced to life in jail in 1995.

The conviction was quashed in 2014 over concerns about the original evidence and a new trial began in June this year.

Mr Eastman's lawyers Ken Cush and Associates in 2015 filed a wrongful imprisonment claim, which was on pause until Thursday's verdict.

Solicitor Sam Tierney said Mr Eastman could have the Australian record for the most time wrongfully spent in prison, but it's difficult to estimate potential compensation.

"How long is a piece of string?," Mr Tierney told AAP on Thursday.

Mr Tierney cited a 2009 case where the WA government issued a $3.25 million ex-gratia payment after Andrew Mallard spent 12 years in jail for a wrongful murder conviction.

Mr Eastman's lengthy retrial ended with gasps from the packed court, as the 73-year-old said "thank you" to the judge after the verdict was delivered.

It took the jury about one week to reach a decision, initially telling the judge it was finding it difficult to come to a verdict.

Mr Eastman's solicitor Angus Webb said "justice has been done", requesting privacy for the 73-year-old.

But the Winchester family are "extremely disappointed" and believe the verdict is wrong.

Mr Winchester was shot twice in the head as he parked on the driveway next to his Canberra home about 9.15pm on January 10, 1989.

The prosecution alleged in the retrial Mr Eastman had developed a murderous hatred of Mr Winchester, who he blamed for imperilling his bid to rejoin the commonwealth public service.

The defence counsel told the ACT Supreme Court there were too many unknowns and gaps for the jury to find Mr Eastman guilty.

Listening devices placed in Mr Eastman's flat revealed him whispering to himself: "He was the first man, the first man I ever killed."

Although he never held an ACT firearms licence, Mr Eastman made numerous attempts to buy guns as far back as early 1988.

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

A witness testified to seeing Mr Eastman at the home of the gun seller, but the defence attacked the credibility of that witness.

The police investigation included a theory of mafia involvement but uncovered no evidence pointing to that.

Terry O'Donnell, who was Mr Eastman's lawyer at the original trial, said the investigation in the 1990s had been an "absolute disgrace".

When asked if the killing was a mafia hit, Mr O'Donnell told reporters: "Put it this way: I have enough evidence to know that it is."

Australian Associated Press