Ambulance officials urged people with respiratory problems such as asthma to stay indoors and aviation officials warned pilots that visibility was down to five kilometres (three miles).

"The conditions for today with all the smoke in the atmosphere are absolutely terrible for people with asthma. It is imperative that they stay inside," said an ambulance official.

The bushfires, most sparked by lightning strikes, have blackened almost 180,000 hectares (450,000 acres) of land, mostly in rugged, inaccessible mountains in the northeast of the state.

Firefighters fear the fires could sweep through some small country towns in Victoria's highlands on Sunday as northerly winds pick up strength ahead of a forecast cool change.

Blazes stretching 150 kilometres (93 miles) from the central King Valley to the Victorian coast could destroy more than 600,000 hectares (1.4 million acres) in coming days as fires merge in the face of strong northerly winds, authorities have said.

"The whole (weather) system has slowed down over the past 24 hours, but we're expecting it to hit tomorrow and when it does it will be severe," said Stuart Ord from Victoria's department of sustainability and environment.

"There is no doubt the fire will hit settlements tomorrow, the question is which ones," Ord told local media.

Army reinforcements have been sent to Victoria state to help more than 2,000 local and New Zealand firefighters.

Firefighters say Australia faces an extreme fire danger this summer after a drought that has turned many rural areas into tinder boxes. Scientists fear climate change will bring more frequent higher temperatures and less rainfall to the country.

Bushfires are a regular feature of Australia's summer. In January 2005, the deadliest bushfires in 22 years killed nine people in South Australia.

Four people were killed and 530 homes destroyed in Canberra in 2003. That same year, bushfires fuelled by drought ravaged a slice of Australia three times the size of Britain.