A seven-day state of emergency has been declared in New South Wales where strong winds have fanned around 100 bushfires.
Around 2,000 firefighters were battling the blazes in Australia, half of which remain uncontrolled, with the support the Australian Defence Force and US and Canadian backup teams.
Rural fire service commissioner Shane Fitzsimmons has said residents should be on alert.
The last state of emergency ran for seven days in mid-November when there was a “catastrophic” fire risk.
It was the first implemented in New South Wales, Australia’s most populous state, since 2013.
Central Sydney reached a maximum of 39C (102F) on Thursday, while outer suburbs had temperatures of 42C (108F).
It came after Australia had its hottest day on record earlier this week.
A statewide total fire ban announced on Tuesday will remain in place until midnight on Saturday.
Around 3 million hectares (7.4 million acres) of land has burnt across Australia in the past few months.
Six people have been killed and more than 800 homes have been destroyed.
Australia’s annual fire season, which peaks during the southern hemisphere summer, started early after an unusually warm and dry winter.
Gladys Berejiklian, premier of New South Wales, said authorities were concerned with the unpredictable conditions.
She said: “With extreme wind conditions, extreme hot temperatures, we have a good idea, a good sense, of where the most concerning areas are, but again when you’ve got those turbulent conditions, embers and spot fires can occur very unpredictably.”
Sydney’s air pollution levels ranged from poor to hazardous on Thursday.
Hazardous smoke has often blanketed Australia’s most populous city and made its iconic skyline barely visible over the past month.
Hospitals have recorded a 10% increase in visits from patients with respiratory conditions during the past week.
The Australian Medical Association has recommended people keep hydrated, cool and out of the sun.
Bushfires are also burning in Queensland, South Australia and Western Australia.
The Bureau of Meteorology said Tuesday was the hottest day on record in the country with an average of 40.9C (105.6F) nationwide.
Perth, the capital on the west coast, is experiencing its hottest December with average temperatures for the month at 36C (97F) and seven degrees above the mean.
Adelaide, in the southeast, is currently experiencing a four-day heatwave culminating in temperatures of 45C (113F).
The unprecedented conditions have reignited debated on whether Australia’s conservative government has taken enough action on climate change.
Australia is the world’s largest exporter of coal and liquefied natural gas.
Protesters camped outside Prime Minister Scott Morrison’s Sydney residence demanding urgent action on climate change on Thursday.
Morrison, who is currently on holiday, conceded last week that “climate change along with many other factors” contributed to the bushfires.