Homicide squad detectives are investigating after four people were taken to hospital on July 30, a day after they became ill following a meal at a home in Leongatha.
A 66-year-old Korumburra woman died in hospital on Friday, while her 68-year-old husband was in a critical condition as of Sunday.
The woman's 70-year-old sister and brother-in-law died on Friday and Saturday, respectively.
Investigators have questioned a 48-year-old Leongatha woman who was released pending further enquiries.
They also executed a search warrant at a home in Leongatha on Saturday.
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 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 wider communities for their "outpouring of love, support, and prayers".
The support extended to their family member who continued to fight bravely in hospital, they said.
"Our families will assist appropriate authorities in any way we can and respect any necessary processes amidst this tragedy," they said.
"As we navigate this challenging time, we kindly request that the privacy of our family, church, and friends be respected. We need space to grieve both privately and as a community."
The families thanked hospital staff who had cared for their loved-ones.
Police investigating the circumstances surrounding the suspected mushroom poisoning and urged anyone with information to come forward.
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