Prosecutors say a judge erred in sentencing mushroom cook murderer Erin Patterson as evidence she will be held in solitary confinement for years to come is lacking.

Justice Christopher Beale handed Patterson, 51, a term of life in prison with a non-parole period of 33 years on September 8, for her murder of three members of her estranged husband’s family, and attempted murder of a fourth.

A jury found her guilty on all counts, after she deliberately poisoned Don and Gail Patterson, and Heather and Ian Wilkinson, with a toxic beef Wellington lunch in July 2023.

On Monday, Victoria’s Director of Public Prosecutions formally lodged an appeal against Patterson’s 33-year minimum prison term. The grounds for that appeal were released on Thursday.

Acting Director of Public Prosecutions, Diana Piekusis KC, said there was an error in the sentence, and Patterson said get a different jail term.

“The sentencing judge erred in finding that there was a ‘substantial chance’ the respondent would be held in ‘solitary confinement for years to come’,” she wrote, in appeal documents released to media.

Piekusis said such a finding was “not open on the evidence, and that finding infected his assessment of the respondent’s likely future conditions in custody and his decision to fix a non-parole period”.

Patterson’s 33-year non-parole period was “manifestly inadequate” and it was in the public interest for prosecutors to bring the appeal, she said.

In a pre-sentencing hearing, prosecutors urged Beale to jail Patterson for life without a non-parole period.

However, he showed Patterson mercy and cited her difficult conditions in custody – including being in solitary confinement for at least 22 hours a day – as part of his reasons for giving her a chance at parole.

“The harsh prison conditions that you have experienced already and the likely prospect of solitary confinement for the foreseeable future are important and weighty considerations,” Beale said on September 8.

“In my view, the only scope for making them count is by the fixing of a non-parole period.”

Patterson continues to deny the three murders and attempted murder and plans to lodge her own appeal against the four jury convictions.

Her new barrister, Richard Edney, last week said Patterson would launch her own legal challenge, although the paperwork was yet to be formally lodged with the Court of Appeal.

In July, a jury found Patterson guilty after a near-11 week trial in the regional Victorian town of Morwell, over the death cap-mushroom laced meal she cooked for her four lunch guests.

Her estranged husband Simon’s parents, Don and Gail, 70, and his aunt Heather, 66, all died in hospital days after the meal, while Heather’s husband Ian Wilkinson survived.

-AAP