Two volunteer firefighters died while battling blazes around Sydney, Australia, authorities said on Friday, forcing Prime Minister Scott Morrison to cut short a vacation in Hawaii as temperatures were again set to soar.

Australia has been fighting wildfires across much of its east coast for weeks, leaving eight people dead, more than 700 homes destroyed and more than 12,000 square kilometres of bushland burned.

As fires ringed Sydney, a fire truck hit a tree and rolled over just to the south of the city, killing the driver and a front passenger, police said. Three other passengers survived with injuries.

"There is no finer person available in my view than someone who is willing to put themselves on the line for the want of nothing in return, no remuneration, nothing other than to make a positive difference in their local community," New South Wales (NSW) Rural Fire Service commissioner Shane Fitzsimmons told reporters.

"There is a definition of hero and these two line up to it."

Fitzsimmons named the dead firefighters as Andrew O'Dwyer, 36, and Geoffrey Keaton, 32. Both men were fathers to 19-month-old children, Fitzsimmons said.

A bushfire burns behind a property in Balmoral, 150 kilometres southwest of Sydney on Thursday. (Peter Parks/AFP/Getty Images)

Earlier, three other firefighters were engulfed by flames as fierce winds fuelled bushfires across the same state. Two men were airlifted to hospital with burns to their faces, arms and legs, while a female colleague was taken by ambulance to hospital.

NSW Premier Gladys Berejiklian said the injured firefighters are now in stable condition

Australia's most populous state declared its second emergency in as many months on Thursday as extreme heat and strong winds stoked scores of uncontrolled bushfires, some on Sydney's doorstep.

In the same week the continent experienced its hottest day on record, thick smoke blanketed the harbour city, shrouded the Opera House and brought many outdoor activities to a halt.

The state of emergency declaration gave firefighters broad powers to control government resources, force evacuations, close roads and shut down utilities across NSW, which is home to more than seven million people.

Firefighters are battling more than 100 fires, more than half of which are uncontrolled, and with temperatures forecast to top 45 C in some areas, officials warned residents to be on high alert.

See Sydney's 'shocking' levels of wildfire smoke, as described by public health lecturer Edward Jegasothy:

Expert says no clear message on how to stay healthy 0:32

The fires have resulted in days of heavy pollution in the city usually known for its sparkling harbour and blue skies.

Many commuters have donned breathing masks in recent weeks as air quality has plunged to hazardous levels not previously seen in Sydney.

"The firefront has been spreading very quickly and intensely," Fitzsimmons told reporters in Sydney, adding that two firefighters had been airlifted to hospital with burns to their faces and airways. "It's still a very difficult and dangerous set of circumstances."

Days out from Christmas, a time when many Australians head to the coast for the holidays, Berejiklian advised people to make sure "you are prepared to change your plans should circumstances change."

The RFS posted footage on its official Twitter account showing firefighters tackling one of the three blazes ringing Sydney. A waterbomber aircraft was dwarfed by thick grey and black billowing cloud as it attempted to douse flames in bushland just metres away from homes.

Watch aerial images of the blazes as Fitzsimmons describes the extent of the damage in one area:

Rural fire head Shane Fitzsimmons describes extent of damage in just 1 area. 0:29

Australia has been battling wildfires across much of its east coast for weeks, leaving six people dead, more than 680 homes destroyed

Some 1,700 firefighters have been deployed across NSW, but officials warned that was still not enough to cover every potential danger and urged people in high risk areas to evacuate while it was still safe to do so.

"There are simply not enough fire trucks for every house. If you call for help, you may not get it," the RFS said. "Do not expect a fire truck. Do not expect a knock on the door. Do not expect a phone call."

EMERGENCY WARNING - Green Wattle Creek (Wollondilly LGA)<br><br>The fire has crossed the railway line near Balmoral. <a href="https://twitter.com/hashtag/nswrfs?src=hash&ref_src=twsrc%5Etfw">#nswrfs</a> <a href="https://twitter.com/hashtag/nswfires?src=hash&ref_src=twsrc%5Etfw">#nswfires</a> <a href="https://t.co/TanlvHhZ6Q">pic.twitter.com/TanlvHhZ6Q</a> —@NSWRFS

The current state of emergency will last for seven days, while a total fire ban that has been in place since Tuesday will remain until midnight on Saturday.

The heightened fire danger in NSW comes as Australia is in the grip of a nationwide heat wave.

The country recorded its highest average maximum temperature of 40.9 C on Tuesday, and Bureau of Meteorology data shows that record was likely to be exceeded again this week.

The extreme conditions have been exacerbated by a warming climate, which is triggering large-scale protests in a country that has been committed to exploiting its vast coal reserves.

PM's holiday stokes anger

The crisis has stoked widespread public anger, with protests around Australia, irritation fuelled this week by Morrison going on leave. He has weathered a storm of criticism on social media, adding to criticism that his government is failing to deliver adequate climate change policies.

As the media reported Morrison was in Hawaii with his family to enjoy a break, about 500 protesters gathered outside his official Sydney residence to demand urgent action on climate change. Morrison's office refused to confirm his whereabouts.

One protester, wearing a Hawaiian shirt, carried a sign reading "ScoMo, where the bloody hell are you?" referencing the leader's nickname and a decade-old international advertisement for Tourism Australia that was banned in several countries because the language was deemed offensive.

A protesters holds up a sign criticizing Prime Minister Scott Morrison's absence during the ongoing wildfires across Australia. (Jenny Evans/Getty Images)

Morrison's office initially declined to comment on the reports, though eventually deputy prime minister Michael McCormack confirmed he was acting leader.

Just hours after confirmation of the deaths of the firefighters, Morrison said he would be returning and issued an apology.

"I deeply regret any offence caused to any of the many Australians affected by the terrible bushfires by my taking leave with family at this time," Morrison said.

Morrison's arrival back in Australia is likely to coincide with temperatures again soaring.

Australia's low-lying Pacific neighbours have also been particularly critical of the coal-rich nation's climate policies following modest progress at the United Nations climate talks in Madrid.

"It was particularly disappointing to see our Pacific cousins in Australia actively standing in the way of progress at a time when we have been watching in horror as their own country is ablaze," Marshall Islands president Hilda Heine said in a statement on Wednesday.