The smoke blanketing Sydney is a "public health emergency," according to a coalition of Australian doctors and researchers who say climate change has helped fuel the wildfires that have produced unprecedented haze.

Air pollution across Australia's most-populous city and parts of the eastern state of New South Wales have reached levels as much as 11 times higher than the threshold for "hazardous," the group said. It called on state and Australian government officials to "implement measures to help alleviate the health and climate crisis."

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Climate change is worsening many extreme weather events, including drought and heatwaves that can predispose devastating wildfires, according to the coalition of 22 health and medical bodies, which includes the Royal Australasian College of Physicians and Public Health Association of Australia.

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At least 2.7 million hectares (6.7 million acres) of New South Wales bushland have burned in the past few weeks, while a blistering heatwave across Western Australia has precipitated dozens of wildfires across the state.

Temperatures in the capital Perth reached 41 degrees Celsius (105.8 degrees Fahrenheit) Saturday and 40 degrees Celsius Sunday, with the same sweltering heatband forecast to move east, reaching Adelaide Tuesday, Canberra Thursday and Melbourne Friday. Brisbane is expected to hit 39 degrees Monday.

Firefighters battles flames from a bushfire in Western Australia. Photo / DFES Evan Collis
Firefighters battles flames from a bushfire in Western Australia. Photo / DFES Evan Collis

South Australia's State Emergency Service issued a community readiness alert, advising residents of steps they can take to keep cool and stay safe. During the heatwave, temperatures may approach 50 degrees on the Nullarbor Plain -- an arid, coastal region straddling Western and South Australia -- and increase the wildfire danger.

There is no safe level of air pollution, the medical groups said. The worse the pollution, the more hazardous the risks to health.

Wildfire smoke is particularly dangerous because of the high levels of tiny particulate matter known as PM2.5. Babies, young children and those who are elderly or already experience chronic respiratory or cardiovascular disease are at higher risk, as are the socially and economically marginalized who may not have access to air conditioning or air purifiers.

NSW Rural Fire Service crews work to protect properties as the Three Mile fire approaches Mangrove Mountain north of Sydney. Photo / Dan Himbrechts
NSW Rural Fire Service crews work to protect properties as the Three Mile fire approaches Mangrove Mountain north of Sydney. Photo / Dan Himbrechts

Air pollution is linked to diseases throughout life, including premature births, low birth-weight babies, impaired lung development in children, asthma, heart disease, stroke, chronic obstructive pulmonary disease and lung cancer.

There's been an almost 2-degree Fahrenheit increase in the global temperature since 1900, while levels of carbon dioxide -- a so-called greenhouse gas -- in the atmosphere have jumped 45 per cent since the industrial revolution, according to the American Association for the Advancement of Science in Washington.

In their statement, Australian health and medical groups called on political leaders to acknowledge the health and environmental emergency of climate change, and "step up and commit to urgent climate action by reducing greenhouse gas emissions in line with what the science demands."

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A kookaburra watches out over a blackened landscape after bushfire swept through Wallabi Point in New South Wales. Photo / Adam Stevenson
A kookaburra watches out over a blackened landscape after bushfire swept through Wallabi Point in New South Wales. Photo / Adam Stevenson

They also called for the development of a national strategy on climate, health and well-being to ensure a nationally coordinated approach to tackling and preparing for the health impacts of climate change, including wildfires, heatwaves, and other extreme weather events.

"Climate change is going to get much worse and we will see more and more of these events," said Fiona Armstrong, executive director of the Climate and Health Alliance. "We must take the kinds of actions on climate change that scientists are calling for -- dramatic cuts in emissions within the next few years -- or we'll see warming potentially spiral out of control and we won't be able to stop it."

Meanwhile, the terrifying bushfire emergency that hit the Blue Mountains late last night happened after the RFS lost control of a "absolutely crucial" backburn, the volunteer fire service admitted.

At least 20 buildings are believed to have been lost last night as the megafire jumped containment lines.

Firefighters had been conducting hazard reduction burns along the southern edge of the Gospers Mountain fire – which has burnt close to 400,000 hectares of bush – when the flames spread.

"There was backburning in the area and unfortunately due to the conditions they're burning in, it's taken one ember to cross over the wrong side and that's where it took a run and it was escalated to emergency warning and then we have had reports of homes lost in the area," an RFS spokeswoman told AAP.

Reports have also surfaced that some firefighters out trying to protect their local communities instead lost their own homes to the megafire.

- With additional reporting from news.com.au