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 the threat of 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 33C 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.
The township of Timber Creek near the WA border southwest of Darwin was hit with heavy flooding, with water flowing through buildings and police rescuing people from the water including a woman in her 70s who was washed off a bridge.
The bureau on Sunday warned people in parts of the Barkly, Simpson and Tanami districts to be prepared for thunderstorms and flash flooding, with "dangerous and life-threatening flash flooding" possible for the Barkly district 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.
Christmas Day forecasts:
Melbourne, 30C max, sunny
Sydney, 28C, Partly cloudy morning, clearing to a mostly sunny afternoon
Darwin, 33C max, high chance of showers and a chance of a thunderstorm
Canberra, 31C max, sunny
Hobart, 25C max, partly cloudy
Adelaide, 32C max, sunny
Perth, 30C max, sunny
Brisbane, 29C max, partly cloudy and a medium chance of showers and a possible thunderstorm
18°C