Video report by ITV News correspondent Neil Connery
The country's Bureau of Meteorology had declared Tuesday the hottest day on record in Australia with an average of 40.9 Celsius (105.6 F) nationwide.
As the extreme conditions intensified, Australia’s most populous state, New South Wales (NSW) declared a seven-day state of emergency in response to the wildfire crisis.
Firefighters continue to tackle around 100 blazes that have already destroyed hundreds of homes and left at least six people dead.
Sydney was blanketed in a plume of hazardous smoke on Thursday, shrouding it iconic skyline.
The heatwave is set to build in some parts of the country, with Friday set to break records across Southern Australian, Victoria and Tasmania, but conditions will ease in Western Australian.
Central Sydney reached a maximum of 39 degrees Celsius (102 Fahrenheit) on Thursday, while outer suburbs scorched at 42 Celsius (108 F).
Around 2,000 firefighters were battling wildfire blazes, half of which remain uncontrolled, with the support of US and Canadian backup teams and personnel from the Australian Defence Force.
A statewide total fire ban announced on Tuesday will remain in place until midnight on Saturday.
Around three million hectares (7.4 million acres) of land has burnt nationwide during a torrid past few months, with six people killed and more than 800 homes destroyed.
The annual Australian fire season, which peaks during the southern hemisphere summer, started early after an unusually warm and dry winter.
NSW Premier Gladys Berejiklian said authorities were concerned with the unpredictable conditions.
Alex Hart from Channel 7 News on the latest on the Australian wildfires
“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,” she told reporters.
Hospitals have recorded a 10% increase in visits from patients with respiratory conditions during the past week.
Wildfires are also burning in Queensland, South Australia and Western Australia.
Western Australia’s capital, Perth, is experiencing its hottest December on record, with average temperatures for the month at 36 Celsius (97 F) – seven degrees above the mean.
The city of Adelaide is currently experiencing a four-day heatwave, reaching 45 Celsius (113 F) on Thursday.