The central point of dispute is whether Senior Constable Miller made mention of one or multiple gunmen.

Barrister Peter Matthews told the Court of Appeal on Tuesday there was a "deliberate manipulation of the evidence of the central issue in the trial, being the number of offenders", and that the conduct of some police verged "to the point of criminal offending, being perjury".

There was a "clear involvement of other officers" in the misconduct, he said, which represented a substantial miscarriage of justice.

"That sort of concealed very serious misconduct poses, we submit, a very serious threat to the integrity of the criminal justice system and to the work of this court," Mr Matthews said.

"It is such an extraordinary case for that, that in order to protect the integrity of the process of this court the entry of acquittals ought occur."

Asked by the appeals judges why a retrial would not suffice if they found in Roberts' favour, Mr Matthews said an acquittal was the fairest outcome, because the police's "conduct is so serious".

"The message needs to be stronger than [a retrial]," he said.

One of the then officers who comforted Senior Constable Miller, Glenn Pullin, said in his statement that he heard his colleague say, "they were on foot".

But it was revealed last year that Mr Pullin's statement from the trial was actually made 10 months after the murders but signed to appear as if it had been made in the hours afterwards.

The doubts over the "dying declaration evidence" was not known by Roberts' barrister, Ian Hill, QC, at the 2002 trial and meant, Mr Matthews said, that he acted with "one hand tied behind his back".

"It is effectively as if there wasn't a trial had," Mr Matthews said on Tuesday.

Roberts' lawyers acknowledge an acquittal would be an extraordinary step for the Court of Appeal to reach but say Roberts only gained the right to an appeal following a series of events which included a whistleblower coming forward and two investigations by Victoria's Independent Broad-based Anti-corruption Commission.

Roberts, 39, has spent two decades in custody since he and Debs were arrested. His lawyers also say Roberts' then girlfriend – Debs' daughter – had in recent years provided an alibi by saying she was with him on the night of the murders.

Ben Ihle, for the Office of Public Prosecutions, said the non-disclosure of revised statements was an irregularity but not a fundamental irregularity.

"The presence of irregularity does not equal an unfair trial. The presence of irregularity does not equal a substantial miscarriage of justice," he said.

Mr Ihle said that on the night of the murders police believed they were looking for more than one gunman and that even putting aside Mr Pullin's statement and the others potentially tainted, there were officers who believed Senior Constable Miller referred to "them".

Conversations between Debs and Roberts caught on listening devices were also "compelling" in the case against the younger man, Mr Ihle said.

And Roberts' initial denials he pulled off armed robberies with Debs – Roberts now admits he committed the hold-ups – were lies that spoke of a "consciousness of guilt".

Mr Ihle said Justice Phillip Cummins, the judge who oversaw the 2002 trial, was moved to remark at the time that he was "affirmatively satisfied" of Roberts' involvement despite his denials.

The Court of Appeal can acquit Roberts or order a retrial if it finds in his favour, or dismiss the appeal. The hearing continues.

With AAP