The heatwave is the worst seen in Victoria, Australia's second-most populous state, since the 2009 Black Saturday bushfires that killed 173 people and is not expected to ease until the weekend, authorities said.

"We are now into day five of the severe to intense heatwave here in Victoria and we are starting to see some of those impacts," said Victorian Emergency Management Commissioner Tim Wiebusch.

"An extreme heat warning is still in place, and we are expecting to see eight consecutive days of severe to extreme intensity heatwave."

David Crock, a forecaster from Australia's Bureau of Meteorology, said the heat was being driven by climate change and called the temperatures "very unusual".

"The severity of this heatwave is comparable to the heatwave in January 2009, and also the heatwave in January 1939," Crock said.

"The data shows a long-term increase in heatwave frequency and intensity, particularly since the year 2000 due to climate change."

Crock said a host of temperature records had been broken in Victoria on Tuesday. "About 20 stations, mostly across western Victoria saw either all-time records or January records fall," he said.

Towns in the Mallee region reached a high of 48.9 degrees Celsius, the hottest temperature ever recorded in the state.

The conditions in Victoria had eased slightly on Wednesday, while the upper western region of the state of New South Wales and south-western Queensland state recorded temperatures above 48 C by the late afternoon.

Around 11,000 properties were without power in Victoria, down from 105,000 a day earlier. Firefighters were also battling six major fires, with three out of control.

A blaze at Carlisle River in the Otways region has burnt more than 11,000 hectares and destroyed at least 16 buildings.

"This fire at Carlisle River is a long way from over," said Chris Hardman, the chief fire officer of Forest Fire Management Victoria.

"We are early in the summer. We'll see the heating of the environment and winds come again well before that fire is fully contained."

Many communities in the state are still recovering from large bushfires at the start of the month, also triggered by a severe heatwave. More than 400 homes and 400,000 hectares of land have been lost so far.

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