Raging bushfires have destroyed at least 10 homes and threatened scores more in southeast Australia as a scorching heatwave hit Sydney with its hottest New Year's Day on record.

Temperatures soared up to 47 degrees Celsius (116 degrees Fahrenheit) as hundreds of firefighters backed by aircraft battled the blazes and sweltering residents of coastal cities flocked to the beaches.

To the north of Sydney, capital of New South Wales and Australia's most populous city, four fires were burning out of control on the central coast, with flames of up to 20 metres (66 feet) high fanned by searing winds.

More than 50 homes in Woy Woy, across the Pittwater estuary from Sydney's upmarket Palm Beach, had been evacuated and at least three houses and seven cars were destroyed, national radio reported.

Helicopters dropped water bombs to help firefighters struggling to bring the blazes under control as they threatened to engulf scores of homes in nearby towns.

"Black smoke covering the sun, just scorching hot, 44 degrees. It's burning up here," a local resident told ABC radio.

Major roads from Sydney to the central coast were closed, stranding holidaymakers in their cars.

Authorities said they would investigate reports that some of the fires may have been deliberately lit.

In Sydney the temperature hit 45 degrees (113 degrees Fahrenheit), causing the failure of a major air conditioning unit at the international airport and leaving sweltering passengers at the mercy of fans and improvised coolers.

"The airport reached 45 degrees at about 4.00pm but the city office at Observatory Hill also hit 44 degrees and that's the hottest New Year's Day ever recorded in Sydney," said weather bureau spokesman Chris Webb.

"The previous warmest was 38.1 back in 1928."

New South Wales fire officials said a total of 44 fires were burning across the state, with some 3,000 firefighters in action or on high alert as weather conditions were expected to worsen later in the day.

"Unfortunately the winds look like they are picking up at this stage and we are expecting them to increase," said spokeswoman Rebel Talbert.

Sydney hospitals had treated a stream of patients affected by the heat, particularly the elderly and those with cardiovascular disease, a spokeswoman for Sydney's health service said.

The heat also caused power failures and delays in city train services, which were hit by faults in overhead wiring on city and regional lines.

Bushfires forced the closure of a section of the central coast rail network, while some 2,500 homes in the area were hit by blackouts.

In neighbouring Victoria state a major fire destroyed seven homes as it swept across a 30 kilometre (18 mile) front, officials said.

The fire has burned through about 9,000 hectares (22,000 acres) of bush and farmland, damaging dozens of properties and leaving two people injured.

The government's bureau of meteorology said recently that average temperatures for the first 10 months of 2005 were 1.03 degrees above the 30-year mean and that the country was on track for the hottest year on record.

The city of Melbourne, capital of Victoria state, topped off its warmest-ever December with a record hot New Year's Eve, the bureau said.

The temperature peaked at 42.9 degrees, breaking the previous record of 41.7 degrees on December 31, 1862.