It's so hot in south-east Australia, even the bitumen in Tasmania is melting.

As the bottom the country withers in the heat, South Australia's heatwave is challenging records dating back more than 70 years and Melbourne is facing its longest run of 40-degree days since 1908.

The South Australian town of Keith recorded the highest temperature in the country on Tuesday with 47.2 degrees as Adelaide sweltered through 45 degrees, 1 degree short of its record set on January 12, 1939.

The heatwave, which ignited destructive fires in Perth, has moved across the country and will give Adelaide five consecutive days above 40 degrees, its third-longest hot spell.

Wednesday is expected to get to 45 degrees and the record may be broken on Thursday with a forecast of 46 degrees before a cool change to 40 degrees on Friday, plummeting to 20 degrees on the weekend.

In Victoria, Charlton and Longerenong were two towns to reach 46 degrees on Tuesday, while Avalon hit 45.8 degrees. Other towns sweated with no power and grass fires burned as the mercury hit 40 degrees in all nine of the Bureau of Meteorology's forecast districts.

Melbourne, which reached 42.8 degrees, will endure four days in a row above 40 degrees after the bureau upgraded Wednesday's forecast to 41 degrees.

It will be the first time the city has endured such heat since 1908, when there was a five-day streak above 40 degrees, Bureau of Meteorology duty forecaster Stuart Coombs said.

''It would go down as the second-longest run of 40s since records started in 1835 in Melbourne,'' he said. ''There will be very little relief during the overnight periods and that's going to make this a very arduous spell of weather for people to get through.''

Several fires flared across South Australia and Victoria on Tuesday and the heat is expected to produce extreme fire conditions by week's end.

Paramedics dealt with 35 cases of heat exhaustion or heatstroke across Victoria.

In normally cool Tasmania, police had reports of melting bitumen as temperatures were about 18 degrees above the average across the state, hitting 40 degrees in Melton Mowbray and Bushy Park, while Strahan reached its second-equal highest temperature of 38 degrees.

Hobart went over the 30-degree mark and will get up to 36 degrees on Friday.

Launceston reached 34.6 degrees on Tuesday and is forecast to hit 38 degrees on Wednesday, remaining in the mid-30s for the rest of the week.

While most of NSW escaped the worst, Swan Hill and Mildura hit 45.2 degrees, and Deniliquin reached 44.3 degrees. AAP