Australians were bracing on 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 (109 F) in Sydney on Tuesday.