Australian towns were among the hottest places on Earth this week as a severe heatwave hit the continent's southeast, with forecasters warning of more record-breaking temperatures before the weekend.

The past four days were among the country's top ten warmest on record, with temperatures nearing 50C in some spots, the Bureau of Meteorology said.

"With South Australia breaking some all-time records yesterday, it would certainly put this region as one of the warmest parts of the world yesterday, if not the warmest," said senior meteorologist Philip Perkins.

"The places that broke records yesterday are already warmer at this time today as they were yesterday."

High temperatures are not unusual in Australia during its arid southern hemisphere summer, with bushfires a common occurrence.

But climate change has pushed up land and sea temperatures and led to more extremely hot days and severe fire seasons.

Among the towns in South Australia state experiencing their hottest temperatures on record on Tuesday was Tarcoola in the region's far north, which reached 49C.

The city of Port Augusta recorded a temperature of 48.9C, almost one degree higher than its previous record set on 7 February 2009.

That was the same day Victoria state endured the devastating "Black Saturday" bushfires that left 173 dead in the nation's worst natural disaster.

The desert town of Coober Pedy, where some residents live underground to escape the harsh conditions, equalled its temperature record of 47.4C.

The past 4 days are in Australia's top 10 warmest days on record—and the trend looks like continuing today. The nights have been warm too, which is what defines #heatwave conditions https://t.co/u6dbfmKPk6 Stay cool, check on loved ones & follow advice from health authorities pic.twitter.com/8Qisw9m4LM — Bureau of Meteorology, Australia (@BOM_au) January 16, 2019

In the state's capital Adelaide, where cycling's Tour Down Under and the cricket One Day International between Australia and India were being held, athletes sweltered through a maximum temperature of 41.9C.

Overnight temperatures were also set to remain high, before a rapid change to cooler conditions from late on Thursday when a cold front over the Southern Ocean sweeps across southern Australia, Mr Perkins said.

"Everything's sort of a bit of a pressure cooker at the moment, and everything's getting hotter and more humid and we're all doing a slow clap waiting for this change to come through," he added.

Australia's creatures, already battered by a drought plaguing the east of the vast continent, are also feeling the heat.

Up to one million fish are believed to have died along the banks of a major river system, with authorities warning of more deaths to come as temperatures soar.

The New South Wales state government said it would install aerators in some waterways to keep the levels of oxygen up for fish to reduce the likelihood of more mass deaths.