Patterson, 51, was in September jailed for life for killing three of her lunch guests by serving them a meal laced with death cap mushrooms.
Prosecutors were pushing for a prison sentence of life without parole but Victorian Supreme Court Justice Christopher Beale showed Patterson mercy by giving her a 33-year non-parole period.
The judge 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 Victorian Director of Public Prosecutions on Monday evening confirmed it would appeal Patterson's sentence.
"We confirm that a DPP appeal has been filed on the basis that the sentence handed down to Erin Patterson is manifestly inadequate," a spokeswoman said in a statement.
Patterson's new barrister Richard Edney indicated last week that his client would also appeal her murder and attempted murder convictions, although the paperwork was yet to be lodged.
Patterson's appeal period was due to expire at 11.59pm on Monday but a new practice note meant appeal extensions would be automatically granted if they were filed with 56 days of sentence.
A jury found Patterson deliberately served beef Wellingtons that were laced with death cap mushrooms to her lunch guests in July 2023 at her Leongatha home in regional Victoria.
Her former in-laws, Don and Gail Patterson, 70, and Gail's sister, Heather Wilkinson, 66, all died after eating the meal, while Heather's husband Ian fell seriously ill but survived.
She was convicted of three counts of murder and one count of attempted murder.
Patterson has always denied the offending, claiming it was a horrible accident.
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