Temperatures are forecast to soar past 40C in parts of southeast Australia, fuelling some of the most dangerous bushfire conditions since the "Black Summer" blazes of 2019-2020.

Country Fire Authority chief officer Jason Heffernan said the fire danger rating in some parts of Victoria state would reach "catastrophic".

"Catastrophic is as bad as it gets," he told reporters.

"It is the most dangerous fire conditions you can expect -- when a fire starts, takes hold and spreads.

"The decisions you make will affect your life and the lives of your family."

Emergency Management Commissioner Tim Wiebush said hot and dry winds would fan bushfires that were "unpredictable, uncontrollable, and fast moving".

Acting Victoria state premier Ben Carroll urged people to prepare evacuation plans.

"You do not know until you are surrounded by fire how loud it is, how smoky, how stressful," he told reporters.

"It is a scary environment that no one should have to go through."

Firefighters are already trying to contain blazes dotted across the states of Victoria and New South Wales.

- Baby bats -

Millions of people across Australia's two most populous states have been warned to remain on high alert, including in major cities Sydney and Melbourne.

Authorities fear a small number of properties have been destroyed near the rural town of Longwood, about 150 kilometres (90 miles) north of Victoria's capital Melbourne.

Government forecaster Sarah Scully said a band of "extreme" heat had settled across the country.

"There's also dry thunderstorms forecast across Victoria and southern New South Wales," she said.

"Those dry thunderstorms have very little rainfall in them, but they can ignite new fires."

Hundreds of baby bats died earlier this week as stifling temperatures hit the state of South Australia, a local wildlife group said.

The "Black Summer" bushfires raged across Australia's eastern seaboard from late 2019 to early 2020, razing millions of hectares, destroying thousands of homes and blanketing cities in noxious smoke.

Australia's climate has warmed by an average of 1.51C since 1910, researchers have found, fuelling increasingly frequent extreme weather patterns over both land and sea.

Australia remains one of the world's largest producers and exporters of gas and coal, two key fossil fuels that are blamed for global heating.