In 2012 the mushrooms killed two Canberra residents who ate them at a dinner party on New Year's Eve and in 2014 they seriously poisoned four others.

There is enough poison in one deathcap to kill an adult and they are not safe to eat, even when cooked.

The Food Safety Information Council issued the warning as the mushrooms began to sprout throughout southern and western Australia.

"These mushrooms start to appear this time of year and have been found in the Canberra region, in and around Melbourne and even in Adelaide," council chair Rachelle Williams said.

"They are not native to Australia and are often found near oak trees growing in warm wet weather during Autumn."

Deathcaps were easily confused with other edible mushrooms and were responsible for nine out of 10 cases of mushroom poisoning in Australia, Ms Williams said.

"Deathcap mushrooms are difficult to distinguish from other wild mushrooms so we recommend you play it safe and only eat mushrooms that you have purchased from the supermarket, greengrocer or other reputable sources.

"People born overseas, especially in Asian countries, should be aware that these deadly mushrooms can look like edible mushrooms that they may have gathered overseas."

Ms Williams said if a person suspected they had eaten a deathcap, they should contact the Poisons Information Centre or go straight to the emergency department.

"Symptoms include vomiting, diarrhoea and stomach cramps and often don't appear until 10 to 16 hours after eating," she said.

"These symptoms may ease for two to three days before a terminal phase of three to four days begins.

"Without early, effective medical intervention, people may go into a coma and die after two or three weeks of liver and kidney failure."

Parents should keep a particularly close eye on young children, Ms Williams said.

"Many reported cases of fungi poisoning are in children under five years of age," she added.

"Most young children who eat poisonous mushrooms find them in the garden at home.

"Children younger than five years of age have a natural inclination to put things in their mouths.

"If you have a toddler, you should regularly check your garden for mushrooms and remove them to reduce the risk of accidental poisoning."

The mushrooms are also poisonous to dogs and other animals.

Canberra residents who believed they had sighted a deathcap should avoid touching it and immediately contact Access Canberra on 13 22 81.

The Poisons Information Centre can be reached 24 hours a day on 13 11 26.