Western Australia's tropical north is about to experience a type of heat that is exceptional even for its standards.

Key points: Temperatures in some towns are forecast to hover in the mid-40s for consecutive days

Temperatures in some towns are forecast to hover in the mid-40s for consecutive days Fitzroy Crossing's forecast 47C maximum will be a record breaker for December

Fitzroy Crossing's forecast 47C maximum will be a record breaker for December The heat will extend in an arc down to Adelaide and Melbourne

Parts of the north will be gripped by an extreme heatwave — the highest intensity there is.

The Bureau Of Meteorology (BOM) defines a heatwave as three or more days in a row in which both day and night temperatures are unusually high for that specific location.

The temperature at Fitzroy Crossing is tipped to reach 47 degrees Celsius on Saturday, which would be a record breaker for December.

Businessman John Rodrigues is accustomed to blistering hot days, having lived in the town for 18 years.

"It's a dry heat so it is bearable, but once it gets to 47C you can really feel it, even in your breathing," he said.

"You don't see that very often, normally we get up to 44C … so it is going to be pretty unbearable especially at night time when it doesn't cool down."

Temperatures at a number of Kimberley locations including Fitzroy Crossing, Wyndham and Kununurra, as well as Marble Bar, Paraburdoo and Telfer in the Pilbara, will hover in the mid-40s for a number of consecutive days, while the nights will range in the high 20s to low 30s.

"People are prepared but you're never going to be ready for it, it's always uncomfortable," Mr Rodrigues said.

"You can see the bitumen — the roads — get really sticky.

"For reprieve, the local people go down to the river and spend the afternoon there cooling down a little bit, so business gets really quiet because no one goes shopping obviously, they're trying to keep cool as much as they can.

"A lot of the locals are elderly people that we need to keep an eye on to make sure they're OK."

What makes it an extreme heat wave?

Heatwaves are classified into three types: low intensity, severe and extreme — which is the rarest kind.

An extreme heatwave affects the reliability of infrastructure such as power and transport, and is dangerous for anyone who does not take precautions to keep cool.

"It's this combination of very high night-time temperatures plus these close to record-breaking, and in some places record-breaking daytime temperatures, that's going to cause the problem," BOM spokesman Neil Bennett said.

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"The body needs to cool down overnight and if you've got minimum temperatures of 28C, 29C, that becomes very very difficult."

Mr Bennett said the heat was being trapped by a slow moving trough of low pressure across the top end.

"We've seen some very hot temperatures through Queensland over the last few days leading to heatwave conditions there," he said.

"This slow-moving trough is basically not going anywhere and so the air mass — the air that's sitting in that region — is being warmed up and quite intensely.

"Until we start to get a little bit of relief with the trough moving, nothing is going to change for the next three days."

The heat will make its way to the other side of the country ahead of the weekend, with Melbourne and Adelaide expecting tops of 38C on Friday.

"We will see this very warm air moving down to the south-east of the country," said Mr Bennett.

"They'll be looking at low intensity heatwave conditions and severe heatwave conditions through parts of northern Victoria initially, then moving down into south-eastern Victoria and even into eastern Tasmania. There's a chance of some severe heatwave conditions."

The Kimberley and Pilbara will swelter under a looming heatwave. ( Supplied: BOM )

Australia's most dangerous natural hazard

Severe and extreme heatwaves kill more people in Australia than any other natural hazard.

The WA Country Health Service advised people to stay out of the heat and keep an eye on vulnerable members of the community.

"Some people are more at risk of developing heat stress, including babies and young children, the elderly, and people with some health conditions or on certain medications," Dr Andrew Jamieson said.

"Symptoms of heat stress include feeling tired or faint, dizziness, feeling thirsty, pale skin and dark urine."

He recommended avoiding alcohol and caffeinated drinks.

"These can increase dehydration," he said.

"Drink plenty of water and limit time outdoors where possible.

"Always remember to seek medical assistance if required. If you're very sick, go to your nearest hospital or call triple zero."