The average temperature in Australia's capital cities is forecast to top 31C with Adelaide set to reach a max of 40C.
Melbourne will not be far behind the South Australian capital with an expected 38C and parts of the state soaring well into the 40s.
Ballarat is tipped to have its hottest November day since 1992.
With Darwin topping out the mercury at 34C and Canberra expected to hit 33C, Perth is just about the only capital in the country to escape the heat with an expected top of 20C.
Brisbane will reach 31C and residents of Western NSW will be subjected to temperatures in the mid 40s.
Relief can be found by the harbour in Sydney where temperatures are only expected to reach a relatively mild 28C.
There are total fire bans in all parts of South Australia due to extreme fire conditions with hot winds expected as part of a major heatwave affecting Victoria, Queensland and New South Wales.
Hobart is forecast to reach 27C.
Near record heat forecast for Victoria
Victoria is facing its first total fire bans of the season as temperatures are set to soar into the 40s.
A fire ban has been declared in the districts and there are fire weather warnings for all western districts of the state.
The says hot northerly winds and humidity of around ten per cent will create severe fire conditions in the west of the state.
Mildura is expecting a top temperature of 44C, which is close to the record high of 45C.
Swan Hill is expecting a top of 43C and the temperature in Melbourne is expected to hit 38C.
Phil King from weather bureau says the conditions are unusual.
"The temperatures we're forecasting today are very close to record hot November temperatures," he said.
He says the heat will only last one day.
"We will see a trough move across the west of the state later this afternoon or evening and then increasing moisture with some rain and thunderstorms developing by Friday morning."
The Country Fire Authority (CFA) has placed extra aircraft in the north-west of the state today as a precaution.
CFA spokesman Brett Boatman is asking farmers in the west and the north-west to delay harvesting until the danger passes.
"We're saying to harvesters and contractors if they can delay their works until the fire danger passes, that reduces the ignition and the chance of fires," he said.
NSW police urge care during hot weather
As NSW braces for extremely hot weather, police are warning motorists about the dangers of leaving children and pets unattended in cars.
The Bureau of Meteorology warns that temperatures are expected to peak in the mid 40s in western NSW on Thursday and will remain in the high 30s and low 40s through the weekend for large parts of the state.
State Emergency Operations Controller, Acting Deputy Commissioner Mark Murdoch, said not only was it dangerous to leave children or animals in cars, it could be deadly.
"There is one golden rule which should never be broken - never ever leave babies, children, the elderly or animals alone in a car even if the air-conditioner is on," he said in a statement.
"It doesn't take long for the temperature inside the car to soar, and for the effects of the heat to take hold."
Mr Murdoch urged everyone in the community to plan ahead, be prepared for the hot weather and beware of the dangers of drinking alcohol and being out in the sun all day.
Police have also urged racegoers attending this weekend's V8 Supercars event in Sydney's west to plan ahead and use public transport as big crowds and heatwave conditions are expected.
Extra police will be patrolling Sydney Olympic Park and surrounding areas for the 2012 V8 Telstra 500 at Homebush.
Assistant Commissioner Frank Mennilli said any alcohol-related offences, anti-social behaviour or illegal drug use would not be tolerated and people should be prepared for the hot weather.
SA faces danger day with total fire bans
There are total fire bans in all parts of South Australia due to forecast heat and extreme fire conditions.
It is the first time in two years there are bans in force for every district, including metropolitan Adelaide.
Temperatures reaching the mid-40s are expected in some areas and strong winds are forecast.
Authorities are also worried about possible lightning strikes later in the day and SA Power Networks is warning people need to be prepared for their electricity to be turned off if there is a danger as the conditions worsen.
Brenton Eden from the Country Fire Service said it was expected to be a challenging day.
"We will rely heavily on the public doing the right thing, reporting any fires that do start and behaving themselves according to the restrictions of a total fire ban," he said.
He said the greatest danger was likely to be until an expected change moved across the state in the afternoon.
"The wind change is about three hours ahead of where it was predicted yesterday, so our major fire weather will be this morning," he said.
"But the concern then is if we get any fire starting, as we have seen in South Australia in the last two weeks, some of these fires can burn for weeks. The risk is if we get an ignition."
Assistant Police Commissioner Bryan Fahy said police would be alert for arsonists.
"We will be pulling out all stops to ensure that people who breach the fire laws will be prosecuted to the fullest extent," he said.
Firefighters are creating a break around at blaze which has burnt about 20 hectares in the state's Murray Mallee, near Pinnaroo.
It was first reported about lunchtime on Wednesday.