A Melbourne woman is critically ill after eating a toxic death cap mushroom.

Six months after a similar outbreak in a Canberra restaurant killed two and sickened one, Australian health authorities are again warning people not to pick their own mushrooms as recent weather conditions have created the ideal environment for the poisonous fungi.

Austin Hospital emergency department director Dr Fergus Kerr said death cap mushroom poisoning is particularly hard to detect as the more severe symptoms may not appear until a day or two after ingestion.

He said poisoning by death cap mushrooms had a mortality rate of about 50 per cent and urged people to only eat commercially farmed mushrooms.