Adelaide sets records for Christmas heat

Adelaide has had its hottest Christmas Day for more than 71 years, becoming the hottest state capital in the country.

The temperature climbed to 40.7C at 3pm in the South Australian capital, the Bureau of Meteorology says, setting a record for the day.

"The last time when we had about 40C on Christmas Day was in 1945 and the temperature back then was 40.1C, so we've broken that record," duty forecaster Kenneth Cheung told AAP.

Other southeastern states also saw the temperature climb into the high 30s, with Melbourne reaching 36C for its hottest Christmas since 1998.

Hobart reached its high of 26.5C early in the day, with the temperature starting to fall after lunch while Sydney saw a high of 27.4C at 2pm.

Meanwhile many Queenslanders had to celebrate indoors with downpours across much of the Sunshine State.

Areas around Noosa had 112mm of rain on Sunday, while Brisbane had about 37mm.

Flood alerts and wild wind warnings are in place for Western Australia's interior as a tropical low crosses the state.

That low is set to head into South Australia by Monday, changing conditions quite dramatically for sweltering Adelaide locals.

There will be an 11C drop in Adelaide's maximum temperature to 29C, the bureau predicts.

A slight cool change will see Melbourne's temperature drop to 31C on Monday while the rest of Victoria faces a heatwave until at least December 29.

The Australian Medical Association has urged people to look after themselves, stay indoors, drink lots of water, and watch out for signs of dehydration.