An Australian pilot has had his conviction for the murder of an elderly camper overturned on appeal and will face a retrial.

Greg Lynn was sentenced last year to a minimum of 24 years in prison for the 2020 murder of 73-year-old Carol Clay at a campsite in Victoria.

But on Thursday the state's Court of Appeal ruled there had been "serious irregularities" during the trial.

"Unhappily, we have concluded that the conduct of prosecuting counsel so compromised the fairness of the applicant's trial that a substantial miscarriage of justice resulted," the panel of three judges wrote in upholding two of Lynn's four grounds for appeal.

In a summary of the ruling, the judges said that in their closing address prosecutors had challenged the evidence given by Lynn and another witness "without ever putting those matters to them" and in doing so "breached a rule of fairness".

They said that in the same address, the prosecution had "unfairly attacked the reliability and credibility of its own firearms and toolmark expert witness" also without giving him a chance to respond.

"In cross-examination by defence counsel, Mr [Paul] Griffiths agreed that aspects of Mr Lynn's version of how Mrs Clay was shot were plausible or 'spot on," the judges wrote.

The breaches were "sufficiently frequent and serious to occasion a substantial miscarriage of justice," and required the court to set aside Lynn's conviction, the judges said.

Lynn, 59, was found guilty of killing Mrs Clay during an altercation in Victoria's Wonnangatta Valley in March 2020 but acquitted over the death of her boyfriend, Russell Hill, 74.

He had pleaded not guilty but admitted to moving the bodies of Mrs Clay and Mr Hill before torching their campsite and later burning their remains.

The case, which sparked one of Victoria's largest missing person operations at the time, gripped Australia.

Lynn was stood down as a pilot by Jetstar - a budget airline owned by Qantas - shortly after his arrest in November 2021.

During his five-week trial last year, Lynn told the jury in Victoria's Supreme Court that he was involved in an altercation with the couple because he was hunting in the area.

His lawyers said that he had then killed the two during two separate struggles for control of a shotgun and a knife - but that it was not murder because the deaths were accidental.

Prosecutors argued he had likely shot Mrs Clay after killing Mr Hill.

His lawyers had also appealed his sentence but the appeal court said that in light of its decision on the conviction, it was not necessary to consider the appeal of his sentence.

Lynn was remanded in custody and will appear in court again on 28 January.