Australian authorities warned Tuesday of some of the worst fire danger since a 2009 inferno which killed 173 people, with most of the continent's southeast sweltering through a major heatwave.

Victoria state, where the so-called Black Saturday firestorm flattened entire villages in 2009 and destroyed more than 2,000 homes, was again bracing for extreme fire weather.

"These next four days promise to be amongst the most significant that we have faced in Victoria since Black Saturday," said acting state premier Peter Ryan.

Tens of thousands of firefighters were on standby, and 1,290 brigades were in a "state of high preparedness", he added, with the peak danger day expected on Friday when very strong winds are forecast.

Two separate grass-fires tested crews early at Little River, west of Melbourne, and Kangaroo Ground to the east.

The flames raced out of control and triggering brief emergency alerts before water-bombing aircraft and engine teams managed to bring them under control.

There were also blazes alight in neighbouring South Australia state.

Victoria and South Australia are bracing this week for what forecasters are describing as "severe to extreme heatwave conditions", with successive days of temperatures above 40 degrees Celsius (104 F) expected.

A similar heatwave struck before the 2009 fires, Australia's worst natural disaster of the modern era in terms of casualties. An estimated 374 people died during the preceding heatwave, with another 173 fatalities in the firestorm itself.

If the forecasts come to pass, Melbourne will endure its longest stretch of hot weather in 100 years.

Road tar was melting in southern Tasmania, with temperatures in the island state some 18 degrees above the January average, breaking several records.

On Tuesday, players at the Australian Open were sweltering.

A ball boy collapsed and water bottles melted on court as the mercury soared above 40 degrees Celsius.

Experts said the outlook had echoes of 2009.

"The forecast weather patterns are quite reminiscent of conditions before Black Saturday, with severe and expansive high temperatures across the southern part of the continent and the presence of low pressure cells on either side of the country in the tropics," said bushfire specialist Jason Sharples from the University of New South Wales in Canberra.

"The combination of high temperature and low relative humidity means that the moisture content of vegetation will be very low. Hence, if a bushfire was to start, it would be expected to spread more rapidly than normal."

Hospitals and emergency authorities are on standby for an influx of heat-related call-outs, with Ambulance Victoria recalling "all available staff (and) every available vehicle."

The heat system has moved across Australia from the west coast, where a wildfire in Perth razed 52 homes on Sunday and claimed the life of one man as he prepared his home for the flames.

Hundreds of residents sheltering in evacuation centres since the weekend were allowed to return to their homes for the first time Tuesday and reported devastating scenes.

"The glass didn't shatter, it melted," said Stoneville resident Stacey Delich.

"It's bad luck and that's all it is," she added. "We live in the bush and we know it can happen, and unfortunately it happened to us."

Wildfires and hot weather are common in Australia's December-February summer months, but the current event is unusual because it is occurring in what is supposed to be a neutral period in the El Nino pattern bringing average conditions.

El Nino, a phenomenon characterised by usually warm ocean temperatures in the central and eastern equatorial Pacific, is generally associated with hotter, drier conditions in Australia.