The Australian state of New South Wales is experiencing one of the highest-risk fire days in its history, with temperatures climbing above 40C.
Firefighters are battling over 100 wildfires raging across the southeast of the country, with more than 20 deemed out of control.
Officials have evacuated national parks, warning that blistering temperatures and high winds are causing "catastrophic" fire conditions in some areas.
All state forests and national parks have been closed as a precaution and total fire bans are in place with temperatures expected to reach as high as 45C in some places.
Strong winds are also forecast, which could fan the flames in unpredictable directions.
Thousands of firefighters are on standby across the nation's most populous state of New South Wales.
NSW Rural Fire Service commissioner Shane Fitzsimmons said: "We are shaping up for one of the worst fire danger days on record.
"You don't get conditions worse than this. We are at the catastrophic level and clearly in those areas leaving early is your safest option."
Australian Prime Minister Julia Gillard urged people to stay safe as she warned the nation to prepare for possibly its worst day of bushfires.
She told Australian television: "This is a very dangerous day.
"We of course are very concerned about these extreme weather conditions in New South Wales. The word catastrophic is being used for very good reason, So it is very important that people keep themselves safe."
One of the worst uncontained fires on Tuesday was around Cooma, about 100km (62 miles) south of the capital city Canberra.
The fire danger in the state of Victoria is also high. The main concern is about a blaze in the southwest, which has already burnt out more than 7,000 hectares of bushland.
No deaths had been reported, although officials in Tasmania were still trying to find around 100 residents who have been missing since a fire tore through the small town of Dunalley, east of the state capital of Hobart, last week, destroying around 90 homes. No bodies were found during preliminary checks of the ruined houses.
Wildfires have scorched 20,000 hectares of forests and farmland across southern Tasmania since Friday.
Bushfires are common during the Australian summer. In February 2009, hundreds of fires across Victoria killed 173 people and destroyed more than 2,000 homes.