Vic swelters through 'blast furnace' heat

Victorians have been urged to stay indoors and avoid the "blast furnace" of what is forecast to be Melbourne's hottest day in nearly two years.

A total fire ban is in place across the state on Saturday, with Melbourne expected to reach 42C.

Mildura is forecast to hit 45C, Warracknabeal 44C, Bendigo and Echuca 43C, and Geelong 42C.

Extreme fire warnings are in place in the Mallee and Wimmera, with severe warnings in the southwest and centre of the state.

Victoria Ambulance warns the "killer" heat would be "like a blast furnace".

"You need to take this seriously. You need to take care. So put off the sporting events, put off the outside events, stay inside," Ambulance Victoria's state health commander Paul Holman told reporters on Friday.

Authorities also warned people not to leave children or pets in cars because they could die within minutes.

"People think they're going to be a minute and they're just going to rush in and grab something, and you're never a minute," Port Phillip Animal Hospital vet Dr Emma Prideaux told reporters on Saturday.

"(A pet's) temperature rises very quickly. They already run at a higher temperature than humans and we do see deaths."

Holiday-makers have been urged to stay on top of changing conditions in their area.

"Extreme weather, extreme heat is as dangerous, more dangerous than any other natural disaster," Agriculture Minister Jaala Pulford said.

On Friday afternoon, a 10km stretch of the Hume Highway near Broadford began melting while a scheduled Saturday horse meet was called off at Mornington.

The hot, dry and windy conditions stretch right across southeast Australia.

Catastrophic fire conditions have been declared for South Australia's Mt Lofty Ranges, and the upper and lower southeast.

Tasmania is also bracing for severe bushfire conditions, while much of NSW is forecast to swelter through temperatures topping 40C over the weekend.

Saturday is forecast to be Melbourne's hottest day since January 13, 2016 before a cool change sweeps through and sends temperatures plummeting by up to 20 degrees.

The Bureau of Meteorology says the cold front in the late afternoon to evening could cause damaging winds averaging between 60 and 70km/h to areas including Melbourne, Geelong, Ballarat, Warrnambool and Bacchus Marsh.