The south-west is expecting a humid, partly cloudy day with temperatures along the coast in the low 20s but much hotter inland.
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There's a 20 per cent chance of a shower in Warrnambool late this afternoon and evening with light winds becoming south to south-easterly 15 to 20 km/h in the middle of the day then becoming even lighter in the late evening.
Warrnambool is expecting a top temperature of 22 degrees, Hamilton and Colac 32, Ararat another 38, Casterton 32, Mortlake 30, Heywood 25, Portland 22 and Port Fairy 21.
On Tuesday Summer heat records were smashed with South Australia's Port Augusta hitting a scorching 48.9 as the rest of the country also sweltered though a heatwave.
The Bureau of Meteorology forecast daytime temperatures of up to 12 degrees above average and 10 higher than usual at night from Monday to Friday.
All states and territories except Tasmania have centres where the temperature is due to stretch into the 40s.
At Port Augusta Tuesday's maximum was the highest since records began in 1962 with 47 forecast for Wednesday and Thursday ahead of a milder Friday.
The small town of Tarcoola in South Australia's far north hit an eye-watering 49 on Tuesday.
"They are pretty incredible temperatures," senior forecaster Michael Efron said.
People in Adelaide didn't escape, with the SA capital reaching 41.2.
The severe conditions in SA also took their toll on cyclists in the Tour Down Under with both the first and second stages shortened due to the heat.
Several towns along the NSW-Victoria border felt the hot weather with Mildura reaching 46.
In central NSW Ivanhoe got to 46.8, while a sea breeze limited inner Sydney to 29 but the western suburbs including Penrith recorded 39.7.
Australian Open players had cooler conditions than on Monday as Melbourne reached a bearable 30.4, while outer suburbs like Scoresby reached 40.2.
A widespread low-intensity heatwave is expected to stretch through to Friday from central Western Australia to southern parts of the Northern Territory and south-western Queensland, as well as parts of Tasmania and Victoria.
Despite comparatively cooler temperatures, Tasmania is facing a severe bushfire threat, with low humidity and some 1200 dry lightning strikes.
