It's been a scorcher for multiple states, but Victoria has been hit hardest by high temperatures.
Two fires raged in a national park and the electricity grid was hit as Victorians sweated through Tuesday, when temperatures surpassed 40C in some parts of the state.
An estimated 500,000 properties were without power as towers collapsed in heat, transmission lines tripped and multiple generators disconnected from the grid.
"These power outages are due to storm activity, strong winds and lightning causing damage to the electricity network," the Australian Energy Market Operator said in a statement.
To keep the network secure and limit further potential damage, reduced supply was rolled out, impacting a further 90,000 properties for a few hours.
Most capital cities surpassed or went close to 30C on Tuesday.
Fire danger and heatwave conditions prompted warnings in South Australia, Western Australia and Tasmania.
Along the coast of NSW the weather soured by late afternoon, with thunderstorms and lightning, although temperatures remained high.
The NSW State Emergency Service responded to 79 jobs - 63 in the greater Sydney area - in the four hours to 7pm.
"We've seen a short, sharp storm with localised impacts," spokesman Ben Deacon said.
The west of Victoria battled high temperatures and thunderstorms, with lightning starting fires and strong winds bringing down powerlines.
Emergency warnings are in place for separate fires in the Grampians National Park, which is in the Wimmera region and is under a catastrophic fire danger warning.
Homes were destroyed and five firefighters suffered injuries when the fire front passed over them.
People near both fires at Bellfield and Mt Stapylton were told to seek shelter from ember and fire attack.
"All agencies have been working very hard over the last couple of days in preparation for today," Emergency Management Commissioner Rick Nugent told reporters on Tuesday.
"We have hundreds of volunteers out on the ground, working incredibly hard to ensure that they get on top of these fires very quickly."
Victoria sweated through its warmest night of the summer and storms started early on Tuesday in the west of the state moving east through the day.
"It's been hot, sizzling," the Bureau of Meterology's Kevin Parkyn, told reporters.
"We've seen 41.7C at Walpeup, 41.4C at Hopetoun there up in the northwest of the state.
"Incredibly 41C at Avalon there ahead of the wind change and the temperature dropped by about 15 degrees down to 26C in 15 minutes."
Some 38 schools and 17 early childhood centres were shut due to Tuesday's dangerous conditions.
An alert remained in place for thunderstorms and some hail for Melbourne and Port Phillip Bay through to the Bellarine and Mornington peninsulas.
A catastrophic fire danger rating was declared for some parts of the state.
It's the first catastrophic rating since the deadly Black Summer of 2019-20.
The bureau forecast extreme fire danger in South Australia's Mid North, Riverland and Murraylands areas.
In Western Australia, a severe heatwave warning covered Kimberley, Gascoyne and Central West Districts.
Tasmania's fire service stopped issuing permits in the south and north of the state until 2am Thursday.
Australian Associated Press