Australian authorities have declared a seven-day state of emergency in New South Wales state as a record heat wave fanned unprecedented bushfires raging across the region.

Australia set a record for its hottest day ever for a second straight day, with an average national temperature of 41.9C, a full degree higher than the previous mark, officials said.

Around 100 fires have been burning for weeks in the state, with half of those uncontained, including a "mega-blaze" ringing Sydney, covering Australia's biggest city in a haze of toxic smoke.

NSW Premier Gladys Berejiklian said the state of emergency, the second declared in the state since an early bushfire season began in September, was due to "catastrophic weather conditions".

There are 2,000 firefighters battling the blazes with the support of US and Canadian teams, as well as Australia Defence Force personnel.

As the heatwave continued, today saw the highest December temperature ever reached in Australia when the West Australian town of Eucla hit 49.8C.

The previous hottest December day was 49.5C in Birdsville, Queensland, in 1972. 

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New South Wales Rural Fire Service commissioner Shane Fitzsimmons said five 100-person "strike teams" were on standby to deploy to the most dangerous fires, "given the enormity of some of these fire complexities and the severity of the forecast weather conditions that are expected to unfold throughout today".

"The worst of the fire weather conditions, the extreme fire danger ratings we are expecting today, are centred around the greater Sydney environment," he added.

Haze has covered Sydney for the past few weeks

The extreme weather is also causing major health concerns, with leading doctors this week labelling the smoke haze that has shrouded Sydney for weeks a "public health emergency".

Hospitals have been recording large increases in emergency room visits for respiratory problems, including a dramatic 80% spike when air quality plummeted on 10 December.