An Australian woman accused of cooking a fatal mushroom meal admits to picking wild funghi, lying to police and disposing of evidence, a court has heard, but will argue the “tragedy” was a “terrible accident”.

The Supreme Court trial of Erin Patterson, 50, began in the small Victorian town of Morwell on Wednesday and is expected to last six weeks.

She is charged with the murder of three relatives and the attempted murder of another, with the case centring on a beef wellington lunch at her house in July 2023.

Ms Patterson has pleaded not guilty and her defence team says she “panicked” after unintentionally serving poison to family members she loved.

Three people died in 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 hospital.

The fact that the lunch of beef wellington, mash potatoes and green beans contained death cap mushrooms and caused the guests’ illnesses is not in contention, the court heard.

“The overarching issue is whether she intended to kill or cause very serious injury,” Justice Christopher Beale said.

Opening the trial on Wednesday, prosecutor Nanette Rogers SC said this case was “originally thought to be a mass food poisoning event”.

But she alleges Ms Patterson “deliberately poisoned” her guests “with murderous intent”, after inviting them for lunch “on the pretence she’d been diagnosed with cancer”.

Dr Rogers said the jury would hear evidence that Ms Patterson had travelled to a location, near her home in Leongatha, where death cap mushroom sightings had been logged on a naturalist website.

And in the days after the lunch, she took a number of steps to “conceal” what she had done, the prosecution alleged.

There’d be evidence that she lied to investigators about the source of the mushrooms in the dish – saying some had come from Asian grocery in Melbourne and she’d never foraged wild ones. And she made a trip to a local dump to dispose of a food dehydrator prosecutors say she used to prepare the toxic meal.

“You might be wondering, ‘What is the motive?'” Dr Rogers said to the jury, “You might still be wondering this at the end of this trial.”

The prosecution will not be suggesting a specific motive, she explained.

“You do not have to be satisfied what the motive was, or even that there was one.”

What the jury could expect to hear, she said, was testimony from a range of witnesses, including: Mr Wilkinson, Ms Patterson’s estranged husband Simon Patterson, medical staff who treated the lunch guests, and police who investigated.

However the defence, in opening their case, reminded the jury they had not heard any actual evidence yet and needed to keep an open mind.

Barrister Colin Mandy says while the prosecution will try to cast Ms Patterson’s behaviour after the lunch as “incriminating”, jurors should consider how someone might react in that situation.

“Might people say or do things that are not well thought out, and might make them look bad?”

“The defence case is that she panicked because she was overwhelmed by the fact that these four people had become so ill because of the food she had served them. Three people died.”

He said Ms Patterson did not deliberately serve poisoned food to her guests.

“She didn’t intend to cause anyone any harm on that day… what happened was a tragedy, a terrible accident.”

The prosecution also detailed allegations of what took place in the lead up to the lunch, and at the table, in open court for the first time.

The trial heard that, in 2023, the accused had been amicably separated from her husband Simon Patterson for years.

“Simon remained hopeful for some time that he and the accused would someday reunite,” Dr Rogers told the jury.

He was also planning to attend the gathering but pulled out at the last minute because he had noticed a recent “change in his relationship” with Ms Patterson and felt “uncomfortable”, the prosecutor said. This was something that “disappointed” Ms Patterson who “emphasised the effort she had put into preparing the lunch”.

The jury was told it would hear testimony that Ms Patterson served her guests on large grey plates, but ate off a different, tan orange dish – prompting one of the guests to later ask if she had “a shortage of crockery”.

They said grace, dug in, and exchanged “banter” about how much they had eaten, before discussing how Ms Patterson should share her cancer diagnosis – which the defence admits was fake – with her children.

The lunch party broke up in the early afternoon, and by that night, all of the guests were feeling ill, Dr Rogers says. Within a day, the four had gone to hospital with severe symptoms. Donald Patterson – who had eaten his portion of lunch and about half his wife’s – told a doctor he’d vomited 30 times in the space of a few hours.

The prosecutor said the Wilkinsons had asked whether Ms Patterson was also in hospital, as she’d eaten the same meal as them.

She had gone to the hospital, reporting feeling ill, but repeatedly declined to be admitted, the court heard. A doctor who had treated the other lunch guests was so concerned for her welfare he called police to ask for help.

Likewise, the jury was told Ms Patterson kept refusing to seek treatment for her children, who she said had eaten the beef wellington leftovers – albeit with the mushrooms scraped off as they didn’t like them.

“Lots of people might have opinions or theories, but they aren’t based on the evidence,” the defence warned the jury at the end of the day.

“None of that should have any bearing on your decision.”