Extreme, record-breaking heatwave conditions are forecast to sweep across four states over coming days, sparking health and fire warnings.
A broad area stretching across much of southern Australia is set to experience the hot weather, with temperatures generally 10C to 14C higher than usual for this time of year, according to the Bureau of Meteorology.
The mercury is expected to soar above 40C in many parts and Mildura, in Victoria's northwest, could see maximum temperatures stay above 40C for four days in a row for the first time in 90 years.
People living in affected areas have been urged to drink plenty of water, avoid travelling during the hottest parts of the day and to look after those most vulnerable to the heat.
Beachgoers hoping to cool off in the sea have also been advised to take care, after four people drowned in coastal waters in two days on Christmas Eve and Christmas Day.
"Summer is well and truly here, with warm to hot conditions developing across southern Australia to finish the year of 2018," meteorologist Jonathan How told reporters on Wednesday.
"We are seeing severe to even extreme heatwave conditions develop across quite a wide area of southern Australia, stretching from Western Australia in the Pilbara district, through South Australia into southern New South Wales and Victoria as well."
Victoria will face an elevated fire risk with total fire bans declared for Thursday in the Mallee and Wimmera regions, while South Australia will have bans in place in 10 areas including the Mount Lofty Ranges and Yorke Peninsula.
Western Australia declared total fire bans in 13 districts on Wednesday and potentially on Thursday when temperatures are expected to exceed 45C in parts.
Meanwhile, Victoria's health department has warned the conditions may lead to increased heat-related illnesses.
Justin Dunlop, state health commander at Ambulance Victoria, said it was important to take seriously the risk posed by the heat.
"We need to look after those around us, our friends, our relatives, those that live alone and our pets as well," he told reporters.
Many parts of Australia are set to experience days of sweltering weather, and the hot conditions could stick until the New Year for Canberra and Sydney.
Canberra could record temperatures above 35C for five consecutive days, while Western Sydney can expect an extended period of above 40C.
Marble Bar in Western Australia is set to hit 49C on Wednesday and might see a run of eight days above 45C for the second time this month.
Highs of 35C in Melbourne and 41C in Adelaide are forecast in the coming days, but the cities should cool over the weekend.
"The heat isn't unusual for this time of year, but what is unusual is how protracted and extended the heat is," Mr How said.
"At the moment the bureau is forecasting a warmer and drier summer than normal, so this could really just be a taste of things to come."
Australian Associated Press
