The other man drove to Sydney after mistakenly consuming the world's most deadly mushrooms.
He took himself to Manly Hospital after experiencing symptoms of poisoning and was transferred to Royal Prince Alfred Hospital on Wednesday.
"The patient is in a critical but stable condition," a health spokesperson said later in the day.
Mycologist Dr Brett Summerell, director of science at the Royal Botanic Gardens, said people often needed a liver transplant after consuming the mushrooms.
"The patients appear to recover after that event, and then a few days later they'll go again into that nausea and vomiting syndrome, plus jaundice," he said.
A fourth man who ate the poisonous meal, believed to have been prepared by the deceased couple, has recovered.
"We believe he had a much smaller serve of the mushrooms," said Dr Michael Hall, director of Canberra Hospital's emergency department.
The chances of dying after consuming "a significant amount" of death cap mushrooms was between 25 and 50 per cent, he added.
"They are not hard to find if you go looking for them," Dr Hall said of the mushrooms that grow readily in the ACT, particularly near oak trees.
The four victims, all of Asian background, may have mistaken the death cap mushrooms for an edible variety, paddy straw mushrooms, which are readily found in Asia.
Dr Hall said people from some cultural backgrounds were more at risk of consuming poisonous mushrooms, because the fungi was a common part of their diet.
Fully-grown death cap mushrooms have silky smooth caps, and the colour varies from white to greenish-brown.
Five people have now died in the ACT from poisoning from death cap mushrooms in the past decade.
In the same period, 15 have been seriously poisoned.
- AAP
