Australians will remain hot and bothered for the rest of the week as heatwave conditions push temperatures into the mid 40s in some regions around the country.

The Bureau of Meteorology has forecast daytime temperatures of up to 12C above average and 10C higher than usual at night from Monday to Friday.

An extreme heatwave is expected to sizzle across most of eastern NSW this week while the rest of the state will experience severe conditions.

In Sydney temperatures are expected to peak on Thursday and Friday with much of the Sydney basin experiencing temperatures 10C above average for this time of the year, a bureau spokeswoman told AAP.

Western Sydney suburbs will swelter with temperatures up to 43C in Richmond and 45C in Penrith on Friday.

Records have already started breaking with Borrona Downs in the state's northwest recording the highest minimum temperature ever in NSW with 34.6C on Monday.

South Australia has had a scorching start to the week with severe heatwave conditions across most of the state and the government declaring a "code red" to unlock extra funds to support homeless shelters.

Severe heatwave conditions are expected for most of SA between Tuesday and Thursday with parts of the pastoral district to hit extreme heatwave conditions.

The hot weather comes as the Tour Down Under cycling event gets ready to start in Adelaide on Tuesday and the Australian Open continues in Melbourne.

Tennis players will have cooler conditions than the cyclists but only just - Melbourne is forecast to experience temperatures in the low to mid-30s all week.

A widespread low intensity heatwave is also expected between Tuesday and Friday from central Western Australia to southern parts of the Northern Territory and southwestern Queensland, as well as parts of Tasmania and Victoria.

TOTAL FIRE BAN IN VICTORIA

A total fire ban has been put in place across Victoria as temperatures are set to soar in the state.

The Bureau of Meteorology is forecasting three straight days of hot to very hot temperatures in most areas of Victoria with the forecast expected to hit the mid-40s in the north of the state.

"Certainly there are extreme conditions for those up in the north of the state," senior forecaster Michael Efron said.

"Temperatures across the border will be around 45C or 46C on both Tuesday and Wednesday."

Mr Efron said it was not a "classic" fire day with northerly winds but instead the southwest winds would pick up in the late afternoon increasing the fire risk.

Planning and preparing ahead of the hot weather was important for everyone, Emergency Management Commissioner Andrew Crisp said.

"We're still in the summer holiday period before school returns, with a large number of people camping and visiting national parks. It is important everyone stays vigilant and don't light camp fires," he said.

"There won't be much relief from the heat overnight on Tuesday, meaning if we have fires in the landscape they could challenge firefighters."

The fire danger is elevated with an extreme warning in the Wimmera, a severe rating for the Mallee, Northern Country and North Central and a very high danger across the rest of Victoria.

SOUTH AUSTRALIA SWEATS

The State Emergency Service has issued an Extreme Heatwave Emergency Warning and the state government has declared a Code Red for Tuesday and Wednesday as temperatures soar towards the high 40s in some parts of the state.

The Code Red triggers extra funding so services for the homeless can be extended while a special phone line will also operate for the next two days, providing regular checks on the elderly and others at risk from the extreme conditions.

Adelaide is forecast to have a maximum temperature of 41C on Tuesday and 40C on Wednesday before cooler conditions push through the state on Thursday afternoon.

The top in the city on Monday also hit 40C while in northern centres like Marree it climbed to 48C.

Premier Steven Marshall has urged all South Australians to take care during the very hot days and stay out of the sun as much as possible.

"These are extraordinary temperatures that are forecast this week," the premier said.

"My strong message to South Australians is to stay hydrated, stay out of the heat as much as possible, and most importantly, check on the vulnerable."

The SES said the extreme conditions, with little relief even at night, would pose a particular risk to babies and young children, the elderly, pregnant women and those who already unwell.

"But in an extreme heatwave, even healthy people who do not take action to keep cool can become very ill," it said.

Amid the searing conditions, the Australia Institute is set to release a report on Tuesday that suggests Adelaide could experience nearly three times as many days over 35C by 2090, unless action is taken to better tackle climate change.

The report, from the institute's Climate and Energy Program, uses CSRIO and Bureau of Meteorology modelling to show how areas in and around Adelaide will get hotter, more regularly.

"Adelaide already has some of the hottest weather of any Australian capital city and, unless we do more to tackle dangerous global warming, that is only going to get worse," the institute's projects manager Noah Schultz-Byard said.

HOW TO STAY SAFE IN A HEATWAVE

- Stay well-hydrated

- Avoid alcohol and hot or sugary drinks

- Limit your physical activity

- Try to stay out of the sun during the hottest part of the day

- Wear light, loose-fitting clothing made from natural fibres like cotton

- Regularly check your forecasts from the Bureau of Meteorology on radio, TV, internet or App

- Get advice from your doctor about whether your medication and/or your medical conditions may affect what you should do if it gets extremely hot

- Make sure you know who you are going to call (who may need help, and who could provide help to you if needed); make a list of telephone numbers and make sure they are current

TEMPERATURES AROUND THE COUNTRY TODAY

Sydney – min 23, max 30

Melbourne – min 19, max 33

Brisbane – min 22, max 32

Perth – min 17, max 31

Adelaide – min 22, max 41

Hobart – min 16, max 29

Canberra – min 19, max 39

Darwin – min 25, max 32

© AAP 2019