The 50-year-old mother is facing her fifth week on trial in regional Victoria, charged with three murders and one attempted murder over a toxic beef Wellington she served up on July 29, 2023.
Patterson has pleaded not guilty to all offences and claims the poisonings were not intentional and it was a terrible accident.
Her former in-laws Don and Gail Patterson, 70, and Gail's sister Heather Wilkinson, 66, died in hospital days after the meal.
Heather's husband Ian Wilkinson was the only surviving lunch guest.
Victorian health department senior adviser Sally Ann Atkinson was called as a witness on Monday afternoon.
She was notified by a doctor, on July 31 2023, that five members of a family had eaten a meal together and four had become unwell from suspected death cap mushroom poisoning.
Ms Atkinson said she "immediately" notified her manager because "this was something that was quite unusual".
"We don't get a lot of amatoxin (toxic mushroom) poisoning, in fact I've never had one before, so it sounded very serious," she said.
"That's something that we needed to alert the food safety unit about, to make sure that we put things in place to try and get those off the shelves."
Ms Atkinson spoke to Patterson on August 1 and she told her she'd experienced "explosive diarrhoea" after eating the meal.
She asked Patterson about the beef Wellington's preparation, and she said "it was a meal she'd never made before and that she wanted to do something fancy", Ms Atkinson said.
Patterson told her the mushrooms were a combination of fresh ones from Woolworths and dried mushrooms from an Asian store which she had purchased in April 2023, from Clayton, Oakleigh or Mount Waverley.
She said Patterson told her she'd bought the dried mushrooms for a pasta dish "but when she opened it she thought they smelled funny" so she put them back into a container.
"The way she explained it indicated she could have used some at that point," Ms Atkinson said.
However, when she spoke to Patterson again, on August 2, she said she gave her different information about the dried mushrooms.
"She stated that the mushrooms had not been used in any other meal," Ms Atkinson said.
"She said that they smelt funny when she'd originally opened them ... so thought that might be too overpowering for the meal she'd originally planned them for."
Ms Atkinson also claimed Patterson initially told her she'd bought other ingredients for the lunch the day before, on the Friday, but that later changed to "Wednesday, Thursday or Friday".
She said Patterson had not responded to several text messages, phone calls and a voicemail from Ms Atkinson between the evening of August 1 and morning of August 2.
This included no response to a text sent by Ms Atkinson with a list of questions for Patterson, including confirming if she'd used the dried mushrooms another time and whether anyone had reported symptoms from that meal.
She said child protective services worker Katrina Cripps helped her make contact with Patterson about 1pm on August 2.
Monash Council investigators searched 14 stores in Oakleigh, Clayton and Mount Waverley, but no mushrooms matching Patterson's description were found, the jury has heard.
Patterson's defence team is yet to cross-examine Ms Atkinson.
The trial will continue with Ms Atkinson on Tuesday morning.
