Christine McKenzie, a Victorian Poisons Information Centre specialist for 17 years, told the Victorian Supreme Court she found the mushrooms growing in Loch in April 2023.
The regional town is about 30 km northwest of Leongatha, where Erin Patterson is accused of murdering her in-laws, Don and Gail Patterson, and Gail's sister, Heather Wilkinson.
It's alleged she deliberately served them a beef Wellington laced with death cap mushrooms on July 29, 2023.
Heather's husband Ian also ate the lunch but survived, and Patterson has been charged with his attempted murder.
She has pleaded not guilty to all charges and is on trial in the Supreme Court at Morwell.
Ms McKenzie told the jury she was walking with her husband and grandson at the Loch Memorial Reserve on April 18, 2023, when she saw mushrooms growing under an oak tree.
Upon closer inspection, she realised they were death cap mushrooms and pulled them out of the ground to photograph.
Ms McKenzie then removed all of the death caps she could find and placed them into a dog poo bag, she told the jury.
"Because of my training, I was very well aware of the toxicity," she said on Monday.
"It's a popular area for people to walk their dogs ... and I was very keen to remove all the samples I could find."
Ms McKenzie said she posted four photos of the mushrooms onto the iNaturalist website that afternoon, along with the exact location where they were found.
She told the jury she had made dozens of posts on the "citizen science" website, mainly about fungi, because mushrooms fascinated her.
Ms McKenzie said she was confident the mushrooms located in Loch were death caps because of her years of experience identifying them at the Poisons Information Centre.
"I feel much more confident about (identifying toxic mushrooms) but as far as the wider fungi kingdom, I'm often as much in the dark as anyone else," she said in evidence.
Ms McKenzie confirmed she did not return to the oval in the days or weeks after spotting the death caps, although there was a risk more could have grown back.
The trial before Justice Christopher Beale continues.
With AAP.
