The fire danger was elevated across wider swathes of southern Australia on Thursday, with residents warned to avoid at-risk areas as smoke from bushfires choked Sydney and other major cities.

Smoke shrouds the Sydney Opera House in Sydney, Australia, November 21, 2019. (Photos: VCG)

Devastating fires along the country's east coast have claimed six lives and destroyed more than 500 homes since mid-October, with climate change and unseasonably hot, dry conditions fuelling the unprecedented blazes.

Now the fire danger has moved into states further south, with a so-called "Code Red" – the highest possible fire risk in Victoria – being declared in the state's northwest for the first time in a decade.

"What that means is that if we see fires in those areas they will be fast moving, they will be unpredictable, they will be uncontrollable," emergency management commissioner Andrew Crisp said.

Firefighters battle a spot fire in Hillville, Australia, November 13, 2019. 

Country Fire Authority chief Steve Warrington told people living in rural areas to leave for the safety of cities. "We are saying, 'do not be there, do not be there when a fire occurs, because you will not survive if you are there'," said Warrington, adding that "there is a good chance if a fire occurs that your home will be destroyed."

The fire danger was also elevated to "severe" in the island state of Tasmania off mainland Australia's southeastern coast, where a total fire ban was declared.

Two bushfires in the state's northeast did not pose an immediate threat to residents, the Tasmania Fire Service said.

For the second time in two days, smoke from bushfires blanketed Sydney, Australia's biggest city and home to more than five million people, sending air quality plummeting to hazardous levels.

More than 110 fires are still burning in worst-hit New South Wales and neighboring Queensland, while in South Australia more than 40 fires broke out during catastrophic fire conditions Wednesday.

A South Australia Country Fire Service spokeswoman said all of those blazes had been brought under control or extinguished by Thursday, with the exception of a major fire on the Yorke Peninsula that had come perilously close to a small town.

A large bushfire burns 20,000 hectares in the Wollemi National Park near Lithgow, 100 kilometers west of Sydney, Australia, November 7, 2019.

Conditions were expected to ease in the coming days in South Australia, where the state capital Adelaide was also shrouded in bushfire smoke and residents were being told to stay indoors for health reasons.

Bushfire-prone Australia has experienced a horror start to its fire season, which scientists say is beginning earlier and becoming more extreme as climate change pushes temperatures higher and saps moisture from the environment after months of severe drought.

The country is bracing for challenging fire conditions to continue throughout the Southern Hemisphere summer.

The fires in southern Australia are the latest in an early start to the Australian bushfire season. 

A total fire ban was put in place in Victoria on Thursday with the temperature forecast to top 40 degrees centigrade in Melbourne for the first time in November since 1997.