Australians were bracing yesterday for what officials warned could be some of the worst fire danger days 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 today. NSW Premier Barry O'Farrell yesterday sounded a grim warning about the forecast record heat and high winds, with large areas of grass and vegetation ripe for burning following 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 fires in Victoria that 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 today, 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 trying to locate some 100 people still unaccounted for. "That's not to say that 100 people 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. No bodies had been uncovered in searches of the worst-hit areas, with 245 properties examined including 90 badly damaged or destroyed buildings.