Roman Quaedvlieg was the chief of ACT Policing when the re-trial of David Eastman for the murder was ordered in 2014.

Mr Eastman was convicted and sentenced to life for the murder after a 1995 trial.

He spent almost 19 years behind bars before his conviction was found to be unsafe and quashed, and he was released from jail.

On Thursday, Mr Eastman, 73, was acquitted of the 1989 murder after a five month re-trial.

Mr Quaedvlieg - one of Mr Winchester's successors - took to Twitter to share his insights into the case, specifically the theory that Mr Winchester had been killed by the Calabrian mafia, after the news of the not guilty verdict broke.

The retrial heard Mr Winchester had previously worked on an operation observing the Calabrian mafia growing cannabis in Bungendore - men who believed they were under protection from authorities.

The cannabis growers were later arrested and put before court, and the theory went that they believed Mr Winchester had betrayed them.

A portion of the mafia evidence during the re-trial was heard in a closed court.

In a series of tweets, Mr Quaedvlieg said, as ACT police chief, he had spent “many spellbound weeks reading the old reports”.

“The case [against Mr Eastman] was always circumstantial, however some of the avenues of inquiry were fascinating, especially the theory that two Calabrian hit men had travelled to Australia to undertake a mafia assassination on Winchester as retaliation for ‘ratting’ them out,” he said.

“When I read the evidence as presented at the 1st trial, my leaning was [Mr Eastman] was guilty but knew it was hard to prove beyond a reasonable doubt. The retrial verdict reflects that view. “

Mr Quaedvlieg said that, when he read all material, he thought “other avenues needed to exhausted to negate them fully.”

“For the record, while I don’t think that avenue of investigation was exhausted to the point of negation, I don’t believe Winchester was a ‘dirty cop’.

“The evidence points to the contrary, but the Italian organised crime entities involved may have wrongly believed otherwise.”

Prosecutors during the retrial had dismissed the mafia theory, saying the murder was the work of an amateur and that Mr Winchester's death served the organisation no purpose.

Meanwhile, there have been no wild celebrations by Mr Eastman - who is truly free for the first-time in almost 30 years.

Mr Eastman became an immediate suspect after the murder, and was questioned by detectives the following day.

Although he was released from prison in 2014, he had been subject to bail conditions since then.

The former Treasury official did not front the waiting media for a jubilant press conference outside the court after the verdict, instead preferring to quietly slip away through a backdoor.

Mr Eastman spoke privately to a number of close supporters immediately afterwards, although those conversations have not been made public.

Through lawyers he politely declined a request for comment by The Canberra Times shortly after the acquittal.

It is unknown how Mr Eastman spent his first full day of freedom in three decades, other than he has kept his head down in preparation for the litigation of a civil claim.

The fallout of the verdict on Friday centred on the decision to pursue the re-trial, with a peak ACT legal body saying it was a waste of taxpayers' money and did not serve justice.

The ACT Bar Association criticised ACT Director of Public Prosecutions Jon White SC's decision to mount the retrial despite an inquiry's recommendation a new trial would "not [be] feasible and would not be fair".

Mr White declined to respond to the association's criticisms.

The full bench of the ACT Supreme Court in 2014 quashed Mr Eastman's 1995 conviction for the murder and ordered he face a re-trial over the assassination.

The court, comprised of Justices Steven Rares, Michael Wigney and acting Justice Dennis Cowdroy, found Mr Eastman did not receive a trial according to law.

The judges said it had not been convinced that a new trial would be unfair and had even been necessary as a strong circumstantial case of murder of a senior police officer existed against Eastman.

The court heard he should not "escape having a jury decide whether or not he is guilty of that crime".

"Weighing all the relevant factors and considerations, we have concluded that the interests of justice require that we order a retrial," Justice Rares told the court in August 2014.

Despite the court's orders, the ultimate decision on whether to conduct a retrial fell to the Director of Public Prosecutions, Jon White, SC.

Mr White, in November 2014, decided Mr Eastman would be re-tried for the 1989 murder.

Bar president Steven Whybrow said there had been little justification in pursuing Mr Eastman the second time.

"A judicial inquiry had raised real questions about the fragile forensic evidence and the effluxion of time had further undermined the prosecution case and the accused’s capacity to challenge it," Mr Whybrow said.

Mr Whybrow said Mr Eastman's age (he turned 73 during this latest trial), the 19 years he already served in custody, and the cost of the retrial all weighed against the prosecution.

"Perhaps most significantly Acting Justice Brain Martin, who undertook the judicial inquiry into the original conviction, specifically expressed the view that even though he thought Eastman was probably guilty, he still had a nagging doubt (probably the same doubt the jury entertained) and in any event a new trial would be neither feasible or fair," he said.

"The slaying of Colin Winchester was of course an abominable and serious crime that justified the significant resources that went into the inquest, police investigation and the original trial.

"The heinous nature of the crime could not alone justify the retrial of the main suspect particularly where a judge has said such a trial would not be fair and where he had a nagging doubt about guilt."