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Many NSW residents are tipped to cop temps in the 40s on Wednesday, while Tasmanians also sweat through unseasonably warm weather.
In Melbourne, the Bureau of Meteorology (BoM) forecast a maximum of 41C Wednesday, which is the second of four days predicted to be above 40C.
If that happens, it will be the first time the city has endured such a heatwave since 1908, when there was a five-day streak over 40C.
At 1.50pm (AEDT), the temperature in Melbourne was just above 40C, with Charlton, in Mallee, recording a scorching 42.9C.
Melbourne's weather was so hot in the city that at the Australian Open on Tuesday a tennis a player hallucinated that he saw Snoopy on court before fainting mid-match, a ball kid collapsed and water bottles melted on court.
A total fire ban is in place for the state, and the Victorian government issued a heat health alert, urging people to stay hydrated and check on the elderly.
Across the border, Adelaide was the hottest of any capital city, recording 42.4C at 1.50pm.
Adelaide faces temperatures above 40C for the next five days, as does most of the state including Renmark and Port Augusta.
The state's hottest weather on Wednesday was at Roseworthy, near Gawler, at 42.6C.
Extreme heat warnings have been issued by South Australia's State Emergency Service (SES) until Thursday.
The temperature hit a mild 19.3C in Hobart, Tasmania, but a total fire ban is in place for the state's northern and southern regions.
In those regions the state's temperatures were into the 30s, well above average even for summer.
The town of Ouse, in the upper Derwent Valley, was the hottest part of the state at 34.8C.
In NSW, Sydney was spared super-hot weather thanks to a coastal sea breeze that kept the temperature under 30C.
The temp in the CBD hit about 28C after lunchtime, with BoM forecasting a max of 30C.
However, it was much hotter in the city's west, with the bureau expecting the mercury to reach 39C at Penrith.
BoM's warned not to expect a cool change until Saturday.
Large parts of the state are suffering extreme heat, with the town of Deniliquin, in the Riverina, tipped to record a number of 44C days.
BoM put the heat wave blanketing the country's southeast down to a "very slow moving, high-pressure system sitting in the Tasman sea".
"It's been directing steady northerly streams for a number of days now, so it's been building a lot of heat in the interior of the continent," a BoM forecaster said.