Temperatures hit a record high in the eastern state of Victoria, preliminary data showed.

"Conditions are heating up across the state, and we're seeing those 40 degree temperatures roll in," said Country Fire Authority chief officer Jason Heffernan.

"Firefighters on the ground will now start to really notice how hot it is getting, and they'll really start to notice the picking up of the winds," he told public broadcaster ABC.

Authorities issued an immediate emergency evacuation warning for hundreds of people in four country towns at risk from the fire in the Otways region southwest of Melbourne.

Residents in another three rural areas nearby were urged to leave as well, with the warning: "Due to extreme weather today, the fire is predicted to rapidly expand and pose a threat to communities."

A total fire ban was imposed on the state, where a total of six major bushfires were burning.

The temperature in the towns of Walpeup and Hopetoun, northwestern Victoria, peaked at 48.9C, according to the Bureau of Meteorology's website.

The temperature nudged past the state's record of 48.8C, but officials said it would need to be verified later to become official.

The heatwave forced the Australian Open to close the roof over its centre court to protect players and fans, with the temperature in Melbourne forecast to hit 45C.

Hot air has settled across swathes of Victoria, South Australia, and New South Wales.

"Prolonged heat can affect anyone and when high heat continues for a number of days it can affect your health," said Victoria chief health officer Caroline McElnay.

The elderly, children and people with underlying health conditions were most at risk, she warned.

"It can cause potentially fatal health problems such as heat exhaustion and heat stroke but it can also trigger events like heart attacks or stroke," McElnay told reporters.

She urged people to stay cool and hydrated, and watch out for symptoms ranging from dizziness to cramps, rapid pulse, a high body temperature and eventually loss of consciousness.