Rain, wind, heat for Aust Xmas Day

Parts of northern Australia are bracing for a cyclone as most of the rest of the country swelters through searing heat this Christmas.

The Bureau of Meteorology has issued a cyclone watch for Top End residents from Port Roper to the Queensland border, including Groote Eylandt.

Gale force winds from the Category 1 storm are forecast to begin lashing the area by Boxing Day afternoon at the earliest.

Local residents have been urged to finalise their emergency plans, cyclone kit preparations and clear any "potential wind-borne missiles" from their properties.

Soaring temperatures and windy conditions on Christmas Day are more of a concern for authorities further south.

Total fire bans have been declared in regions across Victoria, South Australia and Tasmania, with the mercury tipped to hit the high 30s or more in many areas.

Hobart is on track to record its second hottest Christmas, with the 33C predicted just short of the record 34.6C set in 1945.

South Australia's Country Fire Service state co-ordinator Leigh Miller said the state's forecast conditions are the worst he can recall for Christmas Day in almost 30 years.

"We don't need to have catastrophic conditions for bad fires to start," he told reporters.

Slightly cooler conditions will grace NSW and Queensland, including showers in many coastal areas and temperatures in the mid to high 20s in the capital cities.

Varied weather is expected across Western Australia on Friday, with a near-perfect 27C in Perth, possible thunderstorms in the Kimberley region and scorching temperatures of 46C at Marble Bar in the state's Pilbara region.