Former detective commander Richard Ninness has spoken of his lingering concerns about Eastman as his day of judgment finally comes to hand.

Mr Ninness and his team came under fire in the inquiry into Eastman's conviction, including for a surveillance campaign that sometimes "crossed the line" into the realm of the unfair and unlawful.

But the retired detective has maintained his "experienced and dedicated" team acted properly and integrity was their foremost consideration in the years they worked to solve the murder of the ACT police chief.

There is a chance that Eastman will walk from prison on Friday afternoon after more than 19 years behind bars for the murder of Assistant Federal Police Commissioner Colin Winchester in 1989.

His fate sits in the hands of three judges of the ACT Supreme Court, who will make the momentous decision of whether to quash his conviction, quash it and order a retrial, maintain the conviction, or recommend a pardon by the ACT executive.

Mr Ninness, who led Operation Peat, the investigation into Mr Winchester's murder, says he still believes Eastman is guilty and says he should never be let out.

"This is a cold-blooded, premeditated murder and in Eastman's own words, from the covert listening devices, 'it was the most beautiful feeling' in reference to killing another human being," he said, based on his opinion.

"In my view he should never be released."

The retired commander believes he would be a potential target, and said he thinks "numerous individuals" involved in the case needed to be mindful of their personal security should Eastman walk.

Eastman's supporters, including long-time campaigner Terry O'Donnell, have said the former public servant just wants to get on with what is left of his life.

Mr O'Donnell could not be contacted for a response on Thursday.

Eastman's chance at freedom is the result of an inquiry into his conviction, which found a string of problems – chiefly the major flaws with forensic analysis – led to a substantial miscarriage of justice.

The head of the inquiry, Acting Justice Brian Martin, said he was "fairly certain" of Eastman's guilt but had a "nagging doubt".

Mr Ninness and his team were criticised in the inquiry's final report for their surveillance campaign, and also for their inadvertent failure to disclose to prosecutors that forensic expert Robert Collins Barnes was facing internal discipline in Victoria.

The former commander said he was "very disappointed" with the flaws in Mr Barnes' work that could now allow Eastman's conviction to be quashed.

"However, the Robert Barnes of today is not the Robert Barnes of 20 years ago," he said.

Mr Ninness said he thought the inquiry took a narrow view of the overall investigation.

"[Acting Justice Martin] formed the view that there were flaws in the investigation, which I absolutely reject.

"He also highlights that Eastman was most likely guilty but states, due to the passage of time, Eastman could not be retried, yet Martin relies on witness testimony of investigators and scientists some 20 years later to draw his conclusions.

"In my view this is a clear contradiction."

He said he accepted the judge's criticisms as it was "easy to be an observer rather than an operative".

But he said the evidence his team gathered still stands, and he totally disagreed that police crossed the line in their lengthy surveillance operation.

He paid tribute to all who worked on the investigation and made special mention of Gwen Winchester, Mr Winchester's widow, and the Winchester family.

"The Winchester family has been forced to endure 20 years of inquiries and appeals, [and] throughout they have remained strong and supportive of the police investigation," he said.

"Special mention should be made of Gwen Winchester, who has shown enormous strength and courage to carry on after losing her husband in such a tragic way."

Mr Ninness said he believed that Eastman thrived on the intimidation of people in power who lack the courage to stand up to him.

"This is simply another chapter of David Eastman being able to manipulate the system to benefit himself and his own ego. Still today he poses a serious threat to the community," he said.

The full bench of the Supreme Court will hand down its decision at 2.15pm on Friday.