Lawyers for Australian mushroom murderer Erin Patterson say her conviction was undermined by a "catastrophic" mix-up that led to the jury staying at the same hotel as prosecutors and a key witness.
Patterson is serving a life sentence after killing three of her relatives and trying to kill another by serving them a beef Wellington containing death cap mushrooms at her Victoria home in 2023.
Her lawyers also argued some evidence in the trial was too speculative, while evidence crucial to their own case was wrongfully excluded.
At the same hearing, Victoria's Director of Public Prosecutions is appealing against the sentence, arguing a 33-year parole period is "manifestly inadequate".
Patterson's victims include her in-laws Don and Gail Patterson, both 70, and Gail's sister Heather Wilkinson, 66.
Patterson, 51, did not attend the Court of Appeal in Melbourne in person, opting to watch via video link from prison.
Also not present was Patterson's estranged husband Simon Patterson, who was meant to attend the deadly lunch but cancelled at the last minute, in part due to his belief that his wife had been trying to poison him for years.
Heather Wilkinson's husband Ian, the only surviving guest of the lunch, was in court.
During Wednesday's hearing, the appeal judges heard that while deliberating their verdict, jurors stayed in the same hotel as members of the prosecution team, a key police witness, and journalists.
Patterson's barrister Richard Edney said this "catastrophic" situation had "undermined the integrity of the verdicts and requires the quashing of the convictions".
He said a retrial was required "so that justice can not only be done but be seen to be done".
Director of Public Prosecutions Brendan Kissane accepted it was regrettable but "happened, as far as we can tell… by accident".
"The issue is whether there was communication between this jury and any other person," he said.
"The evidence as it stands points towards there being no communication. Once the court accepts that, there's no irregularity in this situation, let alone a fundamental irregularity that goes to the root of the trial."
Patterson's lawyers have also argued a prosecution theory about how the death cap mushrooms were located and foraged should not have been allowed to be put to the jury.
During the trial, prosecutors suggested Patterson went looking for death caps after two sightings near her hometown were reported on the citizen scientist site iNaturalist.
Jurors heard her phone connected to cell towers in both of those areas.
But at Wednesday's appeal, one of her lawyers, Veronika Drago, accused prosecutors of "turning nothing into something".
The mobile phone evidence only "possibly" showed where Patterson had been, Drago said, while there was no evidence that Patterson had ever accessed or seen the iNaturalist posts.
"Speculation was invited by the prosecution," she said, adding that any value the evidence had "was outweighed by its unfair prejudice".
But crown prosecutor Jeremy McWilliams defended the use of a mobile phone tower expert, arguing the jury could use his evidence to decide whether Patterson had an opportunity to source death cap mushrooms, in the way it was alleged.
In other strands of the appeal it was argued that Facebook messages which Patterson sent months before the lunch - in which she was damning of her estranged husband Simon and his parents Don and Gail Patterson - were irrelevant and should not have been allowed.
Patterson's team said the chief prosecutor Dr Nanette Rogers had been protracted and aggressive in a cross-examination "doused with impermissible conduct".
They also claimed the closing address made by the prosecutors during last year's trial was "problematic" identifying 35 complaints.
Asked by appeal judge Justice Peter Kidd why these issues were not addressed at the time, Edney replied the defence lawyers at trial were under pressure and so his team had found additional problems when "the dust [had] settled from the trial".
In documents filed to the court ahead of the hearing, the prosecutors said "guilty verdicts were inescapable".
"The circumstances surrounding the lunch invitation, the serving of the applicant's meal on a different plate, the entirely different medical outcomes for the applicant as compared to the lunch guests, combined with the extensive array of incriminating conduct, pointed overwhelmingly to a deliberate act of poisoning. "
Prosecutors will continue to dispute the appeal arguments on Thursday, and will also appeal against Patterson's sentence.
She is able to apply for parole after 33 years - but Victoria's Director of Public Prosecution says that was "manifestly inadequate" and it was "inappropriate" for the judge to have set any fixed parole period.
The three appeal judges are expected to reserve their decision meaning it could be many weeks before it is known.
