A daughter-in-law cooked the suspected poisonous mushroom meal that killed three people and has put a church pastor in hospital fighting for life.

Homicide squad detectives are investigating after guests at a family lunch at Leongatha in Victoria's east fell ill on July 29.

Four people went to local hospitals the following day and were transferred to Dandenong Hospital and Austin Hospital.

One of the guests, 66-year-old Heather Wilkinson, died in hospital on Friday.

Her husband, 68-year-old Baptist Church pastor Ian Wilkinson, remained in a critical condition at the Austin Hospital on Monday afternoon.

Ms Wilkinson's sister Gail Patterson, 70, and brother-in-law Don Patterson, 70, also died in hospital.

The Pattersons' daughter-in-law, who police say cooked the meal at her home but did not become ill, has been interviewed by investigators.

Outside her Leongatha home on Monday, the woman said she did not know what had happened.

"I didn't do anything," she said. "I loved them and I'm devastated they are gone."

She declined to answer questions about what meals were served to which guests or the origin of the mushrooms.

Her children were also at the home on July 29 but did not eat the meal, Victoria Police Detective Inspector Dean Thomas said.

They are yet to confirm the type of mushroom the guests ate but Det Insp Thomas said the symptoms were consistent with those from eating a death cap.

"We're working closely with (the) Department of Health and of course our poisons medical experts," he said.

He warned Victorians against eating wild mushrooms.

The families of the dead said they had grappled with shock and grief over the past week.

"Our beloved family members, who we will not name at this time out of respect for their privacy, were cherished individuals," the Patterson and Wilkinson families said in a statement.

"They were parents, grandparents, siblings, children and pillars of faith within our community."

"Their love, steadfast faith, and selfless service have left an indelible mark on our families, the Korumburra Baptist Church, the local community, and indeed, people around the globe."

The families extended their gratitude to their wider communities for their "outpouring of love, support, and prayers".

"Our families will assist appropriate authorities in any way we can and respect any necessary processes amidst this tragedy," they said.

South Gippsland mayor Nathan Hersey said the small Korumburra community was in mourning.

Following a service on Sunday, the mayor spoke with the Baptist Church congregation, who have been gathering regularly to pray for the suspected poisoning victims.

"It's hard because we've had a lot of people experience a lot of grief all at once," Cr Hersey said.

"It's shock and it's grief and it's sadness and it's not just with one person that they love, but with three who they loved dearly who (have) passed away and then now another whose in a critical condition."

Baptist Union of Victoria Rev Daniel Bullock said the association was devastated by the loss of the three people who were active members of the Korumburra Baptist Church.

"We continue to pray fervently for the fourth person who is still in hospital," he said.

"Many Baptists across Victoria will join us in these prayers as well as praying for the family, congregation and local community members, who will be impacted deeply by these losses."

The union would continue to support the families involved as well as church members, Mr Bullock said.

Victoria had a spate of death cap mushroom poisonings in 2020, with eight people in hospital at one point. 

Five ended up in intensive care and one died.