The Bureau of Meteorology said the monsoon has arrived in northern Australia, which comprises parts of Queensland, Western Australia and all of the Northern Territory.
"Widespread rain and cooler conditions will develop over northern Australia in coming days as the #monsoon arrives," the bureau said.
The northern Australian monsoon season is generally from December to March but can extend to several months.
Weatherzone, a private weather monitoring service, said southern Australia's unseasonably cold start to summer will end soon and "very hot" weather will seep over South Australia, Victoria and Tasmania just after Christmas for several days.
"Adelaide is forecast to reach 25ºC on Christmas Eve, 32ºC on Christmas Day and 38ºC on Boxing Day. By 27 December, some forecast models suggest that the mercury could be nudging 40ºC," it said.
The bureau said moderate to major flooding around the 1975 flood level is occurring along the Murray Rivers at Euston, Mildura and Wentworth, Moulamein, Barham, Wakool Junction and Boundary Bend in New South Wales and Victoria.
South Australian authorities have revised the peak flow dates for the Murray River at various townships.
Renmark: 21 - 28 December
Berri: 22 December - 02 January
Waikerie: 29 December - 09 January
Swan Reach: 02 - 13 January
Murray Bridge: 03 - 14 January
Keep up to date with the latest forecast from the
Follow the latest changes by checking the
If you are in a life-threatening emergency, call Triple Zero (000). Call the NSW SES on 132 500 and Victoria Emergency Services on 1800 226 226 if you have experienced damage from storms, wind, hail or a fallen tree and if a tree branch is threatening your property or a person's safety.
To access this information in other languages, call the Translating and Interpreting Service on 131 450 (freecall) and ask them to call VicEmergency Hotline.
If you are deaf, hard of hearing, or have a speech/communication impairment contact National Relay Service on 1800 555 677 and ask them to call the VicEmergency Hotline