Patterson, who was sentenced to life behind bars after serving her lunch guests a death cap mushroom-laced meal in July 2023, is fighting to quash her convictions in the Victorian Court of Appeal.
She did not appear at court in person on Wednesday, instead watching the hearing on a video link from the Dame Phyllis Frost Centre.
Dressed in a royal blue jumper, the 51-year-old murderer appeared to read notes and at times closed her eyes as a courtroom full of members of the public and journalists watched on.
Her surviving victim Ian Wilkinson and other members of the Wilkinson and Patterson families were in attendance, more than three years after the fatal lunch.
Patterson's new barrister Richard Edney started his case by attacking how the jury was sequestered during verdict deliberations.
Jury members were required to remain together but ended up staying in the same hotel as the lead investigator and members of the prosecution team.
That decision was "catastrophic" as it could have resulted in inappropriate communication or subconscious influence, Mr Edney said.
When Justice Peter Kidd questioned whether there was any evidence of communication between jurors and the parties, Mr Edney said there was not.
But he maintained it was still a failure of process that had not been explained by the prosecution and should lead to a re-trial.
"It does amount to a fundamental irregularity," Mr Edney said.
Victoria's top prosecutor argued there was no substance to the argument.
"Far from being some catastrophic failure, there is no evidence ... to support this ground," Brendan Kissane KC said.
There was an email from the jury keepers which confirmed there had been no contact between the jurors and any other person, Mr Kissane said.
"What we submit is the system worked as it was intended to work," the prosecutor said.
"We can't speculate that something might have gone wrong."
In Patterson's other grounds of appeal, her lawyers argued she suffered a substantial miscarriage of justice because she was "forced" to give evidence.
Her barrister Veronika Drago pointed to rulings from the trial judge, who did not allow mushroom photos from 2020 found on Patterson's SD card into evidence.
The defence wanted to use those photos to show Patterson always had an interest in foraging for mushrooms, Ms Drago said.
Without the images, there was only one way jurors could hear evidence of foraging, the court was told.
"It compelled (Patterson) to give evidence on this issue," the barrister said.
"That fundamentally altered what she was forced to address while she was giving evidence."
Ms Drago submitted Patterson's right to silence was compromised as a result.
It was also argued Patterson's cross-examination had been unfair and oppressive, with the prosecutor repeatedly asking whether other witnesses were wrong, mistaken or correct.
The prosecution closing address was also "problematic," Mr Edney said.
A jury found Patterson deliberately served beef Wellingtons laced with death cap mushrooms to her former in-laws Don and Gail Patterson, 70, and Gail's sister Heather Wilkinson, 66.
The three guests died after lunch at Patterson's Leongatha home, in regional Victoria, while Heather's husband Ian fell seriously ill from the meal but survived.
Patterson was also convicted of Mr Wilkinson's attempted murder.
The prosecution will continue its response to Patterson's arguments on Thursday before turning to its sentence appeal.
Mr Kissane will argue the convicted killer's 33-year non-parole period was manifestly inadequate.
The appeal hearing before Justices Stephen McLeish, Lesley Taylor and Peter Kidd continues.
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