Christmas Day to mark start of heat wave

East coast Australians thinking about donning the ugly Christmas jumpers may want to think again as warm temperatures hit the capitals across the country.

Sydney, Melbourne, Brisbane and Canberra are all expecting temperatures from the high-20C to the low-30C on Christmas Day.

Across the country Adelaide, Perth and Hobart are expecting sunny skies and maximum temperatures from the mid-20C to low-30C.

In the Top End, Darwin is expecting a storm and a maximum of 32C.

People looking to cool off at Australian beaches are being urged to stay safe and swim between the flags at patrolled beaches.

The past 12 months have been the deadliest across the country's coastline, with 141 coastal drowning deaths. Three in four of those were at least one kilometre away from a lifesaving service.

Surf Lifesaving Australia general manager Shane Daw said people who ended up at unpatrolled locations should be vigilant for rips and other hazards.

"The safest way for all Australians to enjoy our coastline this summer is to find a patrolled beach and swim between the red and yellow flags," he said.

Christmas Day's warmer weather marks the start of a low-intensity heat wave that is expected to sit across the south of the continent, including Tasmania, until Wednesday.

It will also mean a continuation of the hot weather Western Australia has seen in December, sparking several bushfires.

The Bureau of Meteorology has already issued a heat wave warning for WA, with high temperatures gripping parts of the state from the north to the south.

Back east, temperatures are expected to peak as high as 36C in Melbourne and 40C in Adelaide on Tuesday; as high as 28C in Sydney and 32C in Canberra on Monday; and as high as 32C in Hobart on Wednesday.

Despite the warmer spell, flood warnings across NSW and Victoria remain as South Australia braces for the floodwater to flow down the Murray River.

Christmas Day forecasts:

Melbourne, 30C max, sunny

Sydney, 28C, partly cloudy and chance of thunderstorms

Darwin, 32C max, storms

Canberra, 31C max, sunny

Hobart, 26C max, sunny

Adelaide, 32C max, sunny

Perth, 30C max, sunny

Brisbane, 28C max, partly cloudy and a high chance of showers