Homicide squad detectives are investigating after four people had lunch at a home at Leongatha on July 29 and some started to feel sick about midnight that day.
The four people went to local hospitals at Leongatha and Korumburra the following day and were transferred to Dandenong Hospital and Austin Hospital.
A 66-year-old Korumburra woman died in hospital on Friday, while her 68-year-old husband - a Baptist Church pastor - was in a critical condition at Austin Hospital as of Monday.
The woman's 70-year-old sister and brother-in-law, also from Korumburra, died in hospital on Friday and Saturday, respectively.
Investigators interviewed a 48-year-old Leongatha woman, who was a family member and cooked the meal at her home.
She did not become ill and was released pending further inquiries.
Police searched a home in Leongatha on Saturday.
Children were also at the home on July 29 but did not eat the meal, Victoria Police Detective Inspector Dean Thomas said.
"The deaths are really unexplained and we're trying to get to the bottom of it to understand what has actually occurred," he told Melbourne radio station 3AW.
"Of recent time, we have not had any cases like this."
Det Insp Thomas warned Victorians against eating wild mushrooms.
The families of the dead said they were grappling with shock and grief.
"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 published in the South Gippsland Sentinel Times.
"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 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 told AAP.
"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."
Cr Hersey said many people were worried that if the incident was more than an accident, it would raise another level of sadness and concern.
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 in a statement.
"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.
Police are investigating the suspected mushroom poisoning in conjunction with the Department of Health and other experts.
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.
