Australia's southern island of Tasmania has experienced its hottest day since records began, with the capital Hobart sweltering at 41.8 Celsius (107.2 Fahrenheit) on Friday.

The Bureau of Meteorology said the temperature in Tasmania, where records have been kept since the early 1880s, beat the previous high of 40.8 Celsius set in January 1976.

"There are only three or four (days) on record over 40 degrees so it's quite rare," said Melbourne-based Bureau of Meteorology duty forecaster Murray Keable.

Southern Australia has been experiencing a heatwave that has seen bushfires destroy several buildings in Tasmania, and the temperature rise to 45 Celsius in the South Australian capital of Adelaide.

Further inland Wudinna, on the Eyre Peninsula, hit 48.2 Celsius and a string of other South Australian towns topped 47 Celsius.

In Australia's biggest city Sydney, thousands headed to beaches to escape the heat but the temperature was much milder, hitting a maximum of just 29 Celsius in the city centre.