Severe heatwave conditions are forecast in northern and central parts of South Australia, with the state's capital tipped to reach 41C on Tuesday afternoon, the first day over 40C this year.
By 11am, the mercury had already hit 36C in Adelaide with an extreme UV rating of 13 predicted.
SA Premier Peter Malinauskas urged people to check in on their neighbours and particularly the elderly who can be more at risk in extreme heat.
"It's probably a good time for people to remember to look after themselves, look out for their neighbours, but look out for the elderly in particular, it has been a while since we've gone over that 40-degree threshold," he told reporters.
"I hope there's plenty of South Australians who enjoy a warm night and can get to the beach when they get the chance."
People are being urged to stay cool as the mercury rises around the country. (Jono Searle/AAP PHOTOS)
Multiple towns in the mid-north, Flinders and Riverland regions were expected to be over 40C, while Oak Valley in South Australia's north-west interior was tipped to hit 47C.
Bureau of Meteorology spokeswoman Morgan Pumpa said relief was on its way for some, while others would have to wait longer for the cool change.
"The severe heatwave conditions in Queensland, NSW and parts of Western Australia are being caused by a high pressure system and we're awaiting that cold front," she told AAP.
"A trough moving through is going to bring some relief to parts of the south tomorrow."
But places in SA's northwest, such as Coober Pedy, were forecast to stay in the high 30s until at least Friday.
A heatwave warning was also issued for WA, parts of NSW, Queensland and the Northern Territory.
Sydney is expected to reach the mid-30s on Thursday and Friday, while Penrith in the city's west will be closer to 40C.
Demand for electricity has increased as households attempt to keep cool and the Australian Energy Market Operator urged people to conserve energy where possible to reduce the pressure on the grid.
About 40,000 homes across Queensland were affected by power outages on Monday but Energy Minister Mick de Brenni said it wasn't due to a lack of energy supply.
"The was a storm that did roll through some suburbs, we saw 213,000 lightning strikes and the usual sorts of impacts of vegetation across powerlines," he said on Tuesday.
"The second reason was some heat impacts on localised hardware, so a combination of high demand and prolonged heat on some localised hardware saw that hardware trip to protect the system."
Ms Pumpa said people should reduce outdoor activity during the hottest parts of the day as much as possible, check in on vulnerable neighbours and keep an eye on their pets.
"Once we move up in that range of the severity of heatwaves, people do need to take extra actions to stay safe," she said.
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