Christmas Day weather as Australians flock to the beach

Australia's southern states are enjoying a mostly warm and sunny Christmas Day but heavy rain and storms are threatening to dampen celebrations in the north.

People have flocked to beaches or cranked up backyard barbecues to enjoy the balmy weather but elsewhere others face anxious times due to past rains.

South Australians living along the Murray River are bracing for peaking flood levels, with residents advised to consider evacuating as the water rises to levels not seen in a century.

The peak will be lower than first predicted but thousands of properties along the Murray are still facing inundation in coming weeks.

Most of the capital cities are enjoying warm temperatures and clear skies for Christmas Day, with some early cloud coverage in Sydney and Hobart.

A shower or two is possible in Brisbane, according to the Bureau of Meteorology, and storms and a maximum of 33 degrees Celsius are anticipated in the Top End.

The bureau early on Sunday issued a severe weather warning of intense rainfall and high winds associated with ex-tropical cyclone Ellie as it moves south over the central Northern Territory.

It warned people in parts of Barkly, Simpson and the Tanami districts to be prepared for thunderstorms and 'dangerous and life-threatening flash flooding' through Sunday and into Monday.

Damaging wind gusts with peaks in excess of 90km/h are possible over the Barkly district and flood watches are in place for Tennant Creek, Elliott, Ali Curung, Ampilatwatja, Barrow Creek and Renner Springs.

The NT Emergency Service is urging local residents to secure loose items around their homes, avoid driving into floodwaters, have emergency kits ready and to be prepared for power outages.

While Christmas Day is unlikely to break any temperature records, it will mark the start of a low-intensity heat wave that is expected to sit across the south of the continent, including Tasmania, until Wednesday.