The estranged husband of an Australian woman accused of murdering three of his elderly relatives with a meal laced with deadly mushrooms told a court on Thursday their relationship had become increasingly strained in the year before the deaths.

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, in a case that has gripped Australia.

All four fell ill after a lunch of beef wellington, mashed potatoes and green beans, the court has heard. Prosecutors allege the accused laced the meal with highly poisonous death cap mushrooms at her home in Leongatha, a town of around 6,000 people some 135 kilometres from Melbourne.

Erin Patterson denies the charges, with her defence saying the deaths were a "terrible accident."

Repeatedly declined accused's lunch invitations

Erin's husband, Simon Patterson, wept as he told the Supreme Court of Victoria relations between the pair, who had separated amicably in 2015, deteriorated in late 2022, when he listed himself as separated on a tax return.

The decision triggered a system of child support payments, meaning Simon Patterson no longer paid their two children's private school fees directly, he said.

"I was sure she was very upset about that," he told the court.

He repeatedly declined invitations to the accused's house for lunch, including one on July 29, 2023, when the murders are alleged to have taken place, saying he felt uncomfortable about attending.

Accused told husband she'd made 'special meal' for the group

The court heard on Wednesday the accused had lied about having cancer and hosted the lunch on the pretense of discussing the best way to tell her two children about the illness, in what the prosecution said was an effort to ensure the children did not attend and eat the poisoned meal.

The defence does not dispute the accused lied about having cancer.

Text messages between Simon and Erin Patterson read out in court said the accused found his decision not to attend "really disappointing" as she had spent significant amounts of time and money preparing a "special meal" for the group.

Under cross-examination by the defence, Simon Patterson said the accused had shown significant generosity to him and his family, loaning hundreds of thousands of dollars to his siblings in order to purchase or build property. She also had good relationships with the deceased, he added.

The case 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.

The trial is scheduled to run until early June.