Australians in Victoria have been warned they should prepare for property loss or worse as much of the country faces extreme heatwave conditions.

Temperatures on Friday and Saturday are forecast to hit record highs for most states and territories, with Victoria and South Australia in particular bracing for dangerous fire conditions due to strong winds and hot temperatures.

A total fire ban is in place in Victoria and all regions across the state were given a catastrophic or extreme fire danger rating.

Victorians should brace themselves for more property loss or worse, Country Fire Authority (CFA) chief officer Jason Heffernan told the Australian Broadcasting Corporation (ABC) on Friday.

The conditions were extreme yesterday. They're catastrophic today, Heffernan stated.

Temperatures in Melbourne are expected to hit 42°C on Friday, while areas in north-west Victoria may reach 45°C. A bushfire near Longwood, central Victoria, has burnt through nearly 36,000 hectares, with at least 20 homes in Ruffy destroyed.

The main street looks like a bomb's gone off; we've lost a school, said Ruffy CFA captain George Noye. Some properties have lost everything... just absolutely devastating. But thankfully, at the moment, no lives have been lost.

In the Longwood area, three people remain unaccounted for. Authorities are searching for them after fire officials found their home had burnt down.

In South Australia, temperatures are expected to soar up to 46°C, posing threats of fires across the state.

Heffernan urged all Victorians to be on high alert, not just those near active fires, emphasizing that due to the current conditions, many other fires may have ignited.