An Australian woman accused of murdering three of her estranged husband's elderly relatives with a meal laced with poisonous mushrooms wept as she was questioned over expletive-laden messages about the victims on Tuesday, in a case that has captivated the country.

Erin Patterson, 50, is charged with the July 2023 murders of her mother-in-law Gail Patterson, father-in-law Donald Patterson and Gail's sister, Heather Wilkinson, along with the attempted murder of Ian Wilkinson, Heather's husband.

The prosecution alleges she knowingly served the guests Beef Wellington that contained lethal death cap mushrooms at her home in Leongatha, a town of around 6,000 people some 135 kilometres from Melbourne.

Patterson denies the charges, with her defence saying the deaths were a "terrible accident." She faces a life sentence if found guilty.

Appearing as a witness for her own defence, Erin Patterson was questioned on Tuesday by her barrister Colin Mandy about a series of expletive-laden messages sent to friends regarding the Patterson family.

The court previously heard the relationship between the accused and her estranged husband, Simon Patterson, deteriorated shortly before the alleged murders due to a disagreement over child support.

"I wish I'd never said it. I feel ashamed for saying it and I wish that the family didn't have to hear that I said that," she said of the messages that the court has previously heard in the prosecution's case.

"I was really frustrated with Simon, but it wasn't Don and Gail's fault," she told the court through tears.

Erin Patterson is the first witness for the defence after the prosecution rested its case on Monday, following a month of evidence from witnesses, including relatives and medical, forensic and mushroom experts.

The accused began her testimony on Monday afternoon. It is unknown how long she will give evidence for or whether she will be cross-examined by the prosecution.

The trial, which began on April 29, has seen intense interest from Australian and international media, with podcasters, journalists and documentary-makers descending on the town of Morwell, around two hours east of Melbourne, where the trial is being held.

State broadcaster ABC's daily podcast about proceedings is currently the most popular in the country, with two others also high in the charts.

The trial continues.