Flames fanned by strong winds and high temperatures are hampering thousands of firefighters tackling bush fires in south-eastern Australia.
Victoria's fire authority has reported 14 major fires across the state.
The worst is north-east of Melbourne, where there are fears that several fires will merge into a huge blaze.
The fires have burned out more than 200,000 hectares (495,000 acres) of land and left a pall of smoke over Australia's second largest city.
It is so dense that medical authorities have warned people with respiratory problems to stay indoors.
One home was destroyed at Stonyford, in Victoria's south-west, despite the efforts of firefighters working in temperatures of 41.1C - the hottest December day in more than 50 years. Dozens more houses are under threat.
Military personnel have been mobilised to help volunteer civilian fire crews fight the bush fires.
Specially modified planes and helicopters are dumping water on the flames from above.
Environment department spokesman Stuart Ord said crews were being stretched by trying to tackle fast-moving fires on several fronts in extreme conditions.
"There is no doubt the fire will hit settlements, the question is which ones?" he said.
Victorian Premier Steve Bracks has compared the situation to the state's "Black Friday" in 1939, when 71 people died in bush fires.
No-one has died in the current emergency, but Mr Bracks said the danger remained critical.
"We expect the next two weeks are going to be really like standing on our toes, waiting and hoping the communities are protected as the fires rage and move through," he said.
He condemned arsonists who, early on Sunday, lit grass fires on the outskirts of Melbourne that threatened homes before they were brought under control.
"The full force of the law will be brought to bear to find these people, to bring them to justice," Mr Bracks said. "We have increased penalties in place."
Prime Minister John Howard said the federal government was ready to help Victoria cope with the emergency.
Bush fires are common in Australia's hot summer months, but this year's are being described as exceptional.
Nine people died in fires on South Australia state's Eyre Peninsula in January 2005.
In 2003, more than 500 houses were destroyed and four people killed when a huge fire tore through the capital, Canberra.