Some 4,300 firefighters dug fire breaks in Victoria state, the region worst hit this bushfire season, in a bid to protect towns ahead of the forecast arrival of hot, windy conditions on Thursday, authorities said.

The string of blazes that began over a fortnight ago have so far burned out around 7,500 square kilometres of land and claimed 32 houses.

Department of Sustainability and Environment (DSE) spokesman Stuart Ord expected several towns in eastern Victoria would again come under threat when the winds turned northerly on Thursday.

They included the town of Walhalla, which is wedged between a bushfire just a kilometre away and another, deliberately lit, blaze.

"There is no guarantee the fire won't get into the town tomorrow when the winds start to swirl a bit," Ord said.

Meanwhile towns near Mount Buller in the state's north-east were on high alert, including Dargo, which was surrounded by fire on three sides.

However, in something of a reprieve for the state's embattled firefighters, a band of wet weather forecast to cross the state late Thursday was likely to bring the biggest dump of rain since the fires began, according to the DSE.

"It won't put the fire out -- we would need substantial rain to put this fire out -- so what it will do is damp down the fire for some time," Ord said.

Environment Protection Authority senior manager of rural services John Williamson said that smoke haze blanketing Melbourne as a result of the fires had given Australia's second largest city its worst pollution in over 20 years.

"Certainly the smoke has resulted in the poorest air quality ever recorded since we've been recording in Melbourne," he said.

On the island state of Tasmania firefighters battled two major bushfires that burned out of control despite improved conditions.

South Australia's Country Fire Service said that homes were no longer under threat after firefighters contained an inferno police believe was sparked when a car was torched in a forest reserve.

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