An Australian woman on trial for murder says she threw up the toxic mushroom meal which killed her relatives, after binge-eating dessert.

Erin Patterson has pleaded not guilty to four charges - three of murder and one of attempted murder - over the beef Wellington lunch at her regional Victorian house in July 2023.

Prosecutors allege Ms Patterson deliberately served toxic death cap mushrooms, but only to her guests. Her defence team say the contaminated meal was a tragic accident, and argue it had made their client sick too.

On her third day of testimony, Ms Patterson told the court she had only eaten a small part of the lunch and later consumed two-thirds of a cake, before vomiting.

Ms Patterson also admitted she had lied about a cancer diagnosis, which prosecutors say she used to coax the guests to her house, as she was too embarrassed to tell them she was actually planning to undergo weight-loss surgery.

Three people died in the hospital in the days after the meal, including Ms Patterson's former in-laws, Don Patterson, 70, and Gail Patterson, 70, as well as Gail's sister, Heather Wilkinson, 66.

A single lunch guest survived, local pastor Ian Wilkinson, after weeks of treatment in the hospital.

The Victorian Supreme Court trial, which started almost six weeks ago, has heard from more than 50 witnesses and attracted huge global attention.

In the Morwell courthouse, Ms Patterson gave a detailed account of the fatal lunch, saying she had invited her guests under the premise that she wanted to talk about health issues.

The 14-member jury heard that Ms Patterson went through "quite a long process of trying to decide what to cook" for lunch before choosing to make beef Wellington.

The dish - usually prepared with a long strip of fillet steak, wrapped in pastry and mushrooms - was something Ms Patterson's mother made when she was a child, to mark special occasions, she said.

On the morning of the lunch, Ms Patterson recounted frying off some garlic, shallots and several trays of supermarket-bought mushrooms that had been finely chopped in a food processor.

"I cooked that for a very long time," she said. "You've got to get almost all the water out," she added, so the mushrooms won't make the pastry soggy.

"As I was cooking it down, I tasted it a few times and it seemed a little bland to me," she said.

At this point, she decided to add some dried mushrooms that she had bought from an Asian grocer in Melbourne several months earlier and stored in a container in her pantry.

Asked if that container may have had other types of mushrooms in it, Ms Patterson, choking up, said: "Now I think there's a possibility that there were foraged ones as well."

Yesterday, the court heard that Ms Patterson had started foraging for mushrooms in locations close to her Leongatha home in 2020, and her long-standing love for mushrooms had expanded to include wild mushrooms as they had "more flavour".

Ms Patterson told the court she had served up the food and instructed her guests to grab a plate themselves as she finished preparing gravy.

There were no assigned seats or plates, she told the trial.

Mr Wilkinson previously told the trial the guests had each been given grey plates while Ms Patterson had eaten off an orange one.

Under questioning from defence counsel Colin Mandy, Ms Patterson said she didn't have any grey plates, instead listing black plates, white plates and one that was red on top and black underneath.

During the lunch, Ms Patterson said she didn't eat much of her food - "a quarter, a third, somewhere around there" - because she was busy talking.

After the guests left, she cleaned up the kitchen and ate a slice of orange cake Gail had brought and then "another piece, and another piece" before finishing the rest of the cake.

"I felt sick…over-full so I went to the toilets and brought it back up again," she said.

"After I'd done that, I felt better."

Yesterday, the court heard that Ms Patterson had struggled with bulimia since her teens and was prone to regularly binge eating and vomiting after meals.