New secret and highly sensitive evidence casting suspicion on the 'Ndrangheta, or Calabrian mafia, over the killing of the Assistant Federal Police Commissioner was heard during closed hearings of the inquiry into David Eastman's conviction earlier this year.

Speculation about the involvement of organised crime has simmered away since the night Mr Winchester was shot twice in the head outside his home in January 1989.

Organised crime figures involved in the cannabis trade believed they had paid police for protection and that Mr Winchester received the money.

They were later charged over the drugs and, believing they had been double-crossed, were said to have a motive to kill the ACT police chief.

The claims were extensively investigated at the time, but police could not link any particular person to the assassination.

New but untested evidence heard in the Eastman Inquiry this year potentially raised the suspicion of mafia involvement from mere speculation to the "level of a reasonable hypothesis consistent with Mr Eastman's innocence", the Supreme Court said on Friday.

Yet, despite that, internal AFP documents appear to show the agency's unwillingness to reinvestigate.

That was a source of concern for the full bench of the ACT Supreme Court, which published its judgment quashing Eastman's conviction and ordering a retrial on Friday.

"That [concern] was that the documents show an apparent reluctance on the part of the AFP to address, in a serious and professional manner, the newly available material and its potential significance for the issues in the inquiry," the judges wrote.

The AFP documents the court looked at included an internal review of the new confidential evidence by an acting detective sergeant.

He recommended that: "[N]o further action should be taken in relation to the [new information].

"This is in-line with the purposeful and impartial stance the AFP has taken not to reinvestigate matters previously investigated and prosecuted under operation PEAT."

The detective wrote there was no corroborating evidence to support the new information, and as such it would be an "unnecessary diversion from the factual issues surrounding the conviction of David Eastman".

He also wrote that some of the new information "may result in public criticism of the investigation and members involved", should Eastman's conviction be quashed and a re-trial ordered.

Following the acting detective sergeant's report, a superior AFP officer wrote that the new material had been "thoroughly investigated", clearing the other potential offender as a suspect.

That approach was criticised by Justices Steven Rares, Michael Wigney and acting Justice Dennis Cowdroy on Friday.

"There appears to have been a reluctance to treat the new material seriously and an even greater reluctance to conduct any thorough and impartial reinvestigation of the so-called alternative hypothesis in light of the new material," they wrote.

"This reluctance appears to have flowed, at least in part, from a policy position or 'stance' in relation to any re-investigation given that Mr Eastman had already been convicted."

ACT Policing declined to comment on the criticisms on Monday, saying it would be inappropriate to do so given a re-trial of Eastman has now been ordered.

But the full bench found it was up to the AFP to test whether the new mafia evidence was strong enough to be consistent with Eastman's innocence.

"Whether it does so will require a thorough consideration and testing of the new material and a serious and impartial re-investigation of the alternative hypothesis in light of the new material," the judges wrote.

"That does not appear to have occurred to date, at least on the material in evidence before us."