Authorities in Victoria and South Australia are urging people to behave sensibly and watch out for vulnerable friends and family as the southern states swelter through another extremely hot and humid day.

Melbourne and Geelong were forecast to reach 39C on Sunday, with overnight conditions not expected to fall below 27C.

Parts of the state’s north-west, such as Mildura, were expected to hit 44C following at least a week of temperatures into the 40s.

The lack of winds meant there was 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 warned.

“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.

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, Holman warned festivalgoers that drugs, alcohol and extreme heat were “a really dangerous combination”.

A cool change was expected to hit Melbourne on Monday afternoon but conditions were set to remain muggy until early on Tuesday, the Bureau of Meteorology said.

In South Australia, total fire bans were declared for parts of the state. On Sunday afternoon, lives and homes were at risk from an out-of-control bushfire south-east of Adelaide, authorities warned.

A watch and act warning was issued for the fire at Tooperang, near Goolwa and Victor Habor roads, in the southern Mount Lofty Ranges. The fire was burning north and people who were not prepared were advised to leave immediately, the Country Fire Service said.

Adelaide was forecast to reach 42C on Sunday, Whyalla and Murray Bridge 44C and Port Augusta, Marree and Roxby Downs 46C.

It was Adelaide’s second heatwave this month, following similar conditions the week before which led to the hospitalisation of 40 people.

A cool change was set to bring to bring relief later on Sunday ahead of a forecast top of 29C in Adelaide on Monday.

Elsewhere across the state, conditions were expected to reach into the mid-30s but the north of SA would remain very hot.