SYDNEY (AFP) ― Australians were bracing Monday for what officials warned could be the worst fire danger day ever seen, as police searched for some 100 people still missing in ravaged Tasmania.

Prime Minister Julia Gillard urged Australians to be vigilant in coming days, with scorching temperatures predicted in several states and hundreds of blazes already raging.

The danger was most acute in New South Wales, Australia’s most populous state, with the mercury expected to hit 43 degrees Celsius in Sydney on Tuesday.

NSW Premier Barry O’Farrell sounded a grim warning about the forecast record heat and high winds, with a large amount of grass and vegetation ripe for burning due to bumper spring rains.

“Tomorrow is not going to be just another ordinary day,” said O’Farrell. “Tomorrow will be perhaps the worst fire danger day this state has ever faced.”

Thousands of firefighters and about 70 aircraft were on standby, with some 90 blazes already burning ― 20 out of control ― and catastrophic conditions forecast for regions south of Sydney.

Introduced after the 2009 Black Saturday firestorm in Victoria which claimed 173 lives, a catastrophic rating means fires will be uncontrollable, unpredictable and very fast-moving, with evacuation the only safe option.

Extreme conditions were also expected in Victoria state on Tuesday, with fires already burning there.

Some 13 million of Australia’s 23 million people live in Victoria and New South Wales.

The prime minister called on those in danger zones to plan ahead and decide in advance whether they would evacuate or stay and defend their property if fire struck.

“These are extreme events ... we live in a country that is hot and dry and where we sustain very destructive fires,” Gillard said.

“This is the time to be vigilant, and I do particularly want to pass that message to the people of New South Wales as the temperature gauge starts to rise.”

Wildfires destroyed more than 100 homes on the southern island of Tasmania over the weekend and police were hunting for some 100 people still unaccounted for.

“That’s not to say that there’s 100 people that have come to harm, it’s just to say that we really need to focus on those people as a priority,” acting police commissioner Scott Tilyard told reporters.

“Until we’ve had the opportunity to do all the screening that we need to do ... we can’t say for certain that there hasn’t been a human life or more than one human life lost as a result of these fires.”

No bodies had been uncovered in searches of the worst-hit areas, with 245 properties examined including 90 badly damaged or destroyed buildings.

Tilyard said it would be some time before there was any certainty on fatalities, with police working through about 500 missing person reports.

Police said initial investigations of the Tasmania fires suggested that they ignited accidentally as residents who narrowly escaped the fire front described darkness as it approached.

“As the fire came through, what had been a really bright red sky turned absolutely pitch black,” survivor Del Delagarno told ABC radio.

Her home was one of just six in the seaside town of Boomer Bay still standing.

“It was if it was the darkest midnight you’ve ever seen -- it was absolutely horrendous,” she added.

Wildfires are a fact of life in vast but sparsely populated and arid Australia, particularly in the hot summer months between December and February.

As well as in NSW, Victoria and Tasmania, fires were burning in the Australian Capital Territory and Northern Territory, and South Australia and Queensland states.