The term 'heatwave' gets thrown about during Perth's warmer months when temperatures climb into uncomfortable territory, but the city has actually escaped the past three summers without one.

Key points: WA's Department of Health declared the last official heatwave in February 2016

WA's Department of Health declared the last official heatwave in February 2016 In Perth, they're fuelled by high pressure systems stalling in the Great Australian Bight

In Perth, they're fuelled by high pressure systems stalling in the Great Australian Bight Heatwaves kill more Australians than cyclones, floods and bushfires combined

The last time WA's Department of Health declared an official heatwave was in February 2016, when Perth's maximum temperature surpassed 40 degrees Celsius for four days in a row.

The department defines a heatwave as three or more consecutive days when the forecast minimum and maximum temperatures average at least 32C.

Australia's most dangerous natural hazard

Heatwaves are Australia's deadliest natural hazard, killing more people than cyclones, floods and bushfires combined.

According to data from research centre Risk Frontiers, extreme heat accounted for more than half of all listed deaths attributed to natural hazards from 1900 to 2011 nationwide.

Extreme heat claims more lives in Australia than any other natural hazard. ( Supplied: Risk Frontiers )

But WA's chief health officer Andy Robertson said its effects were often underestimated.

"Various communities prepare very well for some of the other disasters, for example in the Kimberley and the Pilbara people are well prepared for cyclones … it's a similar situation in bushfire-prone areas," he said.

"We probably don't plan as well for heatwaves and part of that is we feel that, 'hey, we're West Australians, we're fine, the heat is not going to hurt us, we're used to the heat'."

Dr Robertson said heat-related hospital admissions generally fell into one of two categories.

"One is the direct effects of heat, so people get heat exhaustion and heat stroke," he said.

"But the more insidious effect is on people who have chronic disease, or the very young and the very old who have limited ability to adjust. They become exhausted over that period of time and what we see is an exacerbation of chronic diseases.

"So we see more [people with] renal disease, heart disease, respiratory disease and they end up in hospital and a number of those may unfortunately die."

But it is not just those with chronic diseases and the young and old who are vulnerable. The list is extensive.

"There are other groups that include pregnant women, people who work outdoors, some of the visitors and new arrivals to Australia who may not be used to our climate and who may make poor decisions around being out in the middle of the day or exercising during the day," Dr Robertson said.

Perth's most severe heat event was in February 1955. ( Reuters: Will Burgess, file photo )

"Then there are those who for various reasons aren't able to access air conditioning or other cooling, so the homeless or those who may be socio-economically disadvantaged."

Perth's most severe heat event was in February 1955 when 30 people died.

Where have the heatwaves gone?

In 2012 heatwaves were gazetted as a hazard under WA's Emergency Management Regulations, and the Department of Health was assigned as the agency responsible for monitoring them.

It then developed its current heatwave threshold of three consecutive days where the highest and lowest temperatures each day average at least 32C. This was met each summer from 2011–12 to 2015–16, but not since.

"We've been lucky over the last [few] years in the metropolitan area, we've had hot days but then it's tended to cool down," Dr Robertson said.

"That ability to have a couple of cool days in-between is actually critical because we often don't see [heat-related] cases on day one, we see them on day three or four because of the cumulative effect of heat and not being able to cool down."

In Perth, heatwaves are generated by high pressure systems which stall in the Great Australian Bight for a number of days.

Highs move across Australia from west to east, but stubborn ones can park in the bight. ( ABC News: Kate Doyle )

In the southern hemisphere, winds rotate anticlockwise around highs.

Highs move across Australia from west to east, but on occasion a stubborn high will park in the bight, pushing hot, dry air from the outback onto Perth in east or north-easterly winds for a number of days.

Bureau of Meteorology (BOM) spokesman Neil Bennett said the lack of heatwaves for the past few summers could be attributed to the rapid movement of the highs from west to east, although it is unclear what is driving it.

'What we think has happened over the last couple of years is that the highs themselves have continued their track from west to east, south of the continent," he said.

"But instead of stalling or slowing down in the bight, they've continued to push through, so we don't see the prolonged build up of heat."

The weather bureau's climate outlook is suggesting a hot summer for most of Australia. ( Pixabay: Gerd Altmann )

But that could be about to change, with BOM's latest climate outlook suggesting most of Australia is in for a hot season.

It predicts daytime temperatures from December to February are very likely to be warmer than average for all of Australia, with the exception of western Tasmania.