Authorities in Australia's eastern states of Queensland and New South Wales declared a state of emergency on Monday as the region prepared for "catastrophic" fire conditions.
People in areas deemed at the greatest risk were urged to evacuate before Tuesday when unprecedented hot and windy conditions are forecast. Officials worry the combination of extreme heat and wind will fan fires already burning.
Three people were killed and more than 150 homes were destroyed over the weekend in bushfires in northern New South Wales (NSW) and Queensland.
Officials said the state of emergency - giving firefighters broad powers to control government resources, force evacuations, close roads and shut down utilities - would remain in place for seven days.
Bushfires are common in Australia’s hot, dry summers but the current outbreak, well before the summer peak, has caught many by surprise.
"Everybody has to be on alert no matter where you are and everybody has to assume the worst and we cannot allow complacency to creep in," NSW Premier Gladys Berejiklian told reporters in Sydney.
Australia's most populous city avoided the worst of the weekend conditions, but authorities have raised the forecast for the greater Sydney region to catastrophic fire danger for Tuesday. It is the first time the city has been rated at that level since new fire danger ratings were introduced in 2009.
Temperatures in greater Sydney are set to soar on Tuesday to more than 34 degrees Celsius (93 degrees Fahrenheit), with strong, dry winds.
"Tomorrow is about protecting life, protecting property and ensuring everybody is as safe as possible," Berejiklian said.
Home to more than 5 million people, Sydney is ringed by large areas of bushland. With next to no rain in recent months, much of it is tinder dry.
NSW Rural Fire Service (RFS) Commissioner Shane Fitzsimmons urged people to evacuate before conditions worsened, warning that new fires can begin up to 20km (12 miles) ahead of established fires.
"Relocate while things are calm without the pressure or anxiety of fires bearing down the back door," he said on Monday.
Authorities stressed that even fireproofed homes would not be able to withstand catastrophic conditions.
Fitzsimmons said an additional 400 firefighters on 50 trucks were heading to NSW from the neighbouring state of Victoria to assist local authorities in tackling the 18 fires across the state, two of which have been deemed out of control by authorities.
In February 2009, Australia's worst bushfires on record destroyed thousands of homes in Victoria, killing 173 people and injuring 414 on a day that became known as "Black Saturday".
The current fires come weeks ahead of the Southern Hemisphere summer, focusing attention on the policies of Australia's conservative government to address climate change.
Environmental activists and opposition politicians have used the fires to call on Prime Minister Scott Morrison, a supporter of the coal industry, to strengthen the country's emissions targets.
Morrison declined to answer questions about whether the fires were linked to climate change as he visited fire-hit areas in northern NSW over the weekend.
Deputy Prime Minister Michael McCormack on Monday accused climate activists of politicising a tragedy.
"What we are doing is taking real and meaningful action to reduce global emissions without shutting down all our industries," McCormack told Australian Broadcasting Corporation radio.
"They don't need the ravings of some pure, enlightened and 'woke' capital city greenies at this time, when they're trying to save their homes, when in fact they're going out in many cases saving other peoples' homes and leaving their own homes at risk."
