Victorian health authorities are urging people to behave sensibly, and watch out for vulnerable friends and family as the state swelters through another extremely hot and humid day.
Melbourne and Geelong are forecast to reach 39C on Sunday, with overnight conditions not expected to fall below 27C.
Parts of the state's northwest, such as Mildura, are expected to hit 44C following at least a week of temperatures into the 40s.
The lack of winds means there's only "very high" fire danger for around Melbourne, and central north and western parts of the state, and a "high" fire danger in the east.
The real danger is the heat and humidity, putting partygoers at festivals such as Midsumma, and the very young, elderly and chronically ill at high risk, state health commander Paul Holman warns.
"We're really concerned for the next 36 hours," he told reporters on Sunday.
"Tonight is going to be a really difficult night. People won't have time to recover and tomorrow's still going to be hot and people's houses will remain hot for a number of days."
People should drink plenty of water, stay inside if possible and must not leave pets or children in cars.
Mr Holman also asked people to call triple-zero if they saw vulnerable people such as the homeless in heat distress.
Beachgoers were urged to swim only between the flags following the rescue of 59 people from patrolled beaches over Friday and Saturday.
After a mass drug overdose at a Melbourne dance party on Friday night, Mr Holman warned festival goers that drugs, alcohol and extreme heat were "a really dangerous combination".
A cool change is expected to hit Melbourne on Monday afternoon but conditions are set to remain muggy until early on Tuesday, the Bureau of Meteorology says.