Temperatures are set to soar across Australia this week, with the mercury expected to exceed 45 degrees Celsius in parts of Victoria, South Australia and New South Wales.

Key points: Some outback towns could hit 46C, while the mercury will top 40C in Penrith

Some outback towns could hit 46C, while the mercury will top 40C in Penrith Organisers of the Santos Tour Down Under will modify the race route in response to forecast conditions

Organisers of the Santos Tour Down Under will modify the race route in response to forecast conditions In far west New South Wales, the town of Menindee is bracing for further fish kills

The Bureau of Meteorology has forecast severe or extreme heatwave conditions for parts of central Western Australia, Victoria, New South Wales, South Australia and southern Queensland into the week.

Low intensity heatwave conditions are also forecast across most of the country.

Temperatures are expected to hit 46C in the town of Menindee today, sparking fears of another mass fish kill, while the mercury is set to tip 40C in the western Sydney suburb of Penrith from Tuesday through to Friday.

Victoria's north is set to swelter through a potentially record-breaking heatwave, with towns along the Murray River bracing for a run of days above 40C, including a forecast of 46C for communities between Mildura and Albury Wodonga on Wednesday.

Heatwave severity Low intensity The level of heat expected is unusual, but injury to people is not generally expected unless inappropriate activities are conducted or sensible precautions are not undertaken. Severe Vulnerable people are at risk of injury. Extreme Likely to cause impact across multiple areas such as infrastructure, transport, energy and agriculture. Healthy and vulnerable people are at risk of injury. 'Vulnerable' includes people already impacted by some form of physical or mental ill-health, the socially isolated, and those in substandard housing or with no access to air conditioning. This group often includes the elderly.

In South Australia, the outback town of Oodnadatta will reach a top of 47C on Monday, while those in Adelaide will be offered little reprieve, with temperatures set to sizzle at 39C.

The Bureau had previously warned of extreme heatwaves forecast to sweep through parts of Australia over the Christmas and New Year Period, with average temperatures up to 12C higher than usual.

Tour Down Under shortened, tennis players bracing for heat

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Due to extreme weather conditions, including severe wind forecasts and rising temperatures, organisers of the Santos Tour Down under have removed a 3.4-kilometre finishing circuit on stage one, scheduled for Tuesday, while the distance of the second stage between Norwood and Angaston, slated for Wednesday, will be reduced by 26.9 kilometres.

"The safety and welfare of the riders, spectators and everyone involved with the race is always our primary concern," said race director Mike Turtur.

No further modifications to race routes are expected.

Meanwhile, Melbourne's two days of 35C maximums coincide with the first two days of the Australian Open, which has introduced a new heat stress scale this year for players.

The five-step scale allows organisers to take different levels of action in response to rising heat on the tennis court.

Under the new system, 10-minute breaks will be allowed in men's singles matches for the first time in a grand slam tennis tournament.

The scale is based on data on the maximum heat stress an athlete can safely withstand, the sweat rate of players and their core temperatures.

Menindee bracing for fish kills

As temperatures soar to 45C in far west NSW, the State Government is warning people to brace for more mass fish deaths and "a lot of poor water quality situations".

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The mercury is expected to reach up to 46C in the town of Menindee, where up to 1 million fish died in an algal bloom over the New Year.

The Bureau of Meteorology said a heatwave, caused by hot air being blown from Central Australia, would persist until Saturday and could break temperature records around Broken Hill.

Primary Industries Minister Niall Blair said state and local governments would work with the community to manage the possibility of another ecological disaster.

"We've got high temperatures right across the state and a lot of poor water quality situations, particularly brought on by the extended drought, so unfortunately we are expecting that we may see more fish killed," Mr Blair said.

The warning comes as contractors prepare to clear a 40-kilometre stretch of the Darling River of dead fish before their rotting carcasses compound the situation.