Justice Paul Coghlan also called Katia Pyliotis' murder defence a "red herring calculated to mislead" and told lawyers that's what he'd tell the jury.

Pyliotis was jailed for 19 years last year for the murder of lonely widower Elia Abdelmessih, whose bludgeoned body was found alongside a tin of mangoes and a Virgin Mary statue in 2005.

Victoria's Court of Appeal on Wednesday overturned the conviction and ordered she stand trial again for the crime.

She faced three failed trials before being convicted in the fourth.

"It will be a matter for the Director of Public Prosecutions whether the applicant faces trial for a fifth time," three judges ruled.

Her barrister, Dermot Dann QC, said the court should overturn the conviction because of Justice Coghlan's "negative and scathing assessment" of the defence case.

At one point while trial lawyer Richard Edney was questioning a witness, Justice Coghlan interjected to tell him "this is even more boring than the other parts of your cross-examination".

Three appeal judges found those comments and others may have unfavourably influenced the jury's consideration of the defence case, which involved a confession to police by another woman who has since died.

"To describe counsel's cross-examination as 'boring' ... had the tendency to demean counsel's competence and to run the risk of diminishing his standing in the eyes of the jury," the judges said.

"Comments such as these - if they are ever justified - should be reserved for occasions when the jury is absent."

But the judges ruled that while the boring comment did not cause a miscarriage of justice, four other comments about the defence case did.

The defence had argued prosecutors could not rule out another woman, Susan Reddie, had killed Mr Abdelmessih.

She confessed to murder but a policeman gave evidence she had recanted to him days later.

Only one of the officer's colleagues had a vague recollection of the recanting of the confession and the police diaries the officer said he recorded it in were lost.

No DNA matching Ms Reddie, who died in 2012, was found at the crime scene, but Ms Pyliotis' DNA was found.

Justice Coghlan told the jury that if there was an alternative suspect, or anyone else in the house, they didn't leave DNA or fingerprints.

"Well whoever that someone else is, it would not seem possible that it was Susan Reddie," he said.

Justice Coghlan also said the evidence about Ms Reddie recanting her confession was "a distraction".

"I think it's all a big red herring ... I make it clear to you that's exactly what I'm going to tell this jury when the moment comes, that it's a giant red herring," he said.

But the appeal judges said whether or not Ms Reddie had recanted was manifestly important.

Prosecutors had also appealed the sentence, urging the appeal court to increase her time behind bars.

Australian Associated Press