It was the second time Mr Eastman faced trial over the murder, and the verdict marks the latest and last chapter in a long-running saga that has consumed the Canberra justice system for 30 years.

At about 9.15pm on January 10, 1989, Mr Winchester pulled into his neighbour's Deakin driveway after visiting his brother in Queanbeyan.

As Mr Winchester got out of his car, an assassin approached under cover of night and the noise of the car radio and unloaded two bullets in the officer's head.

He was the highest ranking police officer ever to be assassinated in Australia, and the AFP famously described his death as the "end of innocence" in Australia.

The federal police turned the full force of their resources to the investigation of their respected chief's murder.

Mr Eastman, a disgruntled public servant who had recently met with Mr Winchester in his efforts to avoid an assault charge, quickly became a suspect.

But he wasn't charged with the murder until December 23, 1992, at the end of the massive police investigation and an inquest that ran for more than three years.

Mr Eastman was found guilty by a jury in the first trial in 1995, and spent 19 years in jail until his conviction was quashed in 2014.

A judicial inquiry had found Mr Eastman had suffered a substantial miscarriage of justice, due in part to flaws in the forensic evidence used against him.

As the foreman read the verdict on Thursday an audible gasp came from the public gallery.

Mr Eastman, who was sat in the accused's dock for the first time this year, stood and bowed to the jury and mouthed the words "Thank you".

When the jurors left the court the judge told Mr Eastman: “I now formally discharge you.”

Mr Eastman's reply: “Thank you.”

The 73-year-old faced an anxious wait after the jury on Monday reported a deadlock. But the judge encouraged the members to continue their deliberations and they returned a verdict at 10.15am on Thursday.

The judge formally discharged the jury shortly after, thanking the members for their service.

They will have the choice to never serve on a jury again if they wish.

The 12 men and women deliberated for more than 31 hours over seven days, after beginning their discussions about 10.30am last Wednesday.

Mr Eastman had pleaded not guilty to the murder, in both 1995 and 2018.

They had sat through 58 days of evidence, 127 Crown witnesses, and six weeks of closing addresses during the marathon murder trial that lasted some five months.

Outside court, lawyers for Mr Eastman said justice had been done.

His solicitor, Legal Aid ACT’s Angus Webb, told the waiting media that a board of inquiry into the conviction in 2014 recommended there be no retrial.

“That recommendation was not adopted, and the ACT DPP determined to proceed with a retrial,” he said.

“Today, five months after the retrial commenced on the 18th of June this year, the jury returned a verdict of not guilty.

“Justice has been done.”

Mr Webb said Mr Eastman wished to thank his current and past lawyers and counsel.

"In particular, Mr Eastman wishes to acknowledge the tireless work done by his retrial lawyers led by Mr George Georgiou of senior counsel.

"Mr Eastman now looks forward to getting on with the rest of his life and asks that his privacy be respected."

On Thursday, the Director of Public Prosecutions Jon White issued a brief statement on the case.

"After a trial in which both parties presented their cases fully and professionally, the justice system has delivered its verdict.

"I would like to compliment the prosecution team led by Murugan Thangaraj SC for their dedication and professionalism. I would also like to acknowledge the investigating officers in this most challenging investigation.

"My thoughts are with the Winchester family who have displayed great forbearance in the face of unimaginable tribulations. There will be no further comment at this time."

Mr Eastman's first trial in the ACT Supreme Court began on May 2, 1995.

That jury had found him guilty of murder, though the trial was characterised by Mr Eastman repeatedly sacking his lawyers and his vitriolic outbursts.

The former treasury official was jailed for life and spent nearly 19 years in custody for the killing.

During that time he protested his innocence all the way to the High Court of Australia, where his attempt to appeal was knocked back.

He was eventually released on August 22, 2014, following a hard-fought-for judicial inquiry.

Acting Justice Brian Martin found in his inquiry that Mr Eastman had suffered a miscarriage of justice and he recommended the conviction be quashed, saying a retrial would neither be feasible nor fair.

He also said while he was fairly certain Mr Eastman was guilty of the murder "a nagging doubt remains".

Prosecutors in the retrial put a circumstantial case to the jury, arguing there were simply too many coincidences for the killer to be anyone other than Mr Eastman.

The Crown also said he had a powerful motive to kill.

Mr Eastman, a former treasury official, had spent a decade in a desperate battle with the bureaucracy for a return to the public service after leaving because of ill health.

But Mr Eastman was furious with police after officers charged him with assault - he believed wrongly - following a dispute with a neighbour and when Mr Winchester refused to intervene.

Mr Eastman believed a conviction for assault, the prosecution said, would permanently exclude him from rejoining the public service.

Prosecutors had evidence Mr Eastman had said he wanted to kill the "bastard police" and told his doctor they should be taught a lesson.

They also had recordings of Mr Eastman alone in his flat muttering to himself, and allegedly confessing to the murder of Mr Winchester.

But the defence disputed Mr Eastman’s whispered mutterings were anything to do with the murder, and challenged the prosecution motive.

They also offered an alternative, that it was a reasonable possibility someone else, namely members of the Calabrian mafia or 'Ndrangheta, had ordered the assassination.

The group believed they and their illicit activities were under protection from authorities, and even Mr Winchester himself. But Mr Winchester was not a corrupt cop.

Eleven members of the group known as the Bungendore 11 were later arrested and put before court, leading them, the defence argued, to believe Mr Winchester had betrayed them.

Prosecutors in turn questioned why the mafia would buy a gun in a Queanbeyan backyard and why the shooter would use both a silencer and noisy supersonic bullets.

Police never charged anyone connected with the mafia. It was Mr Eastman who was put before the court.