State of emergency declared in Australia as bushfires destroy at least 20 homes

New South Wales declared a state of emergency on Thursday after about 20 homes were destroyed in bushfires that have been raging through the Australian state for weeks.

It came as Australia experienced its hottest-ever day on Wednesday, reaching an average maximum temperature of 41.9C and smashing the 40.9C record set just one day earlier.

Temperatures of 46C and above were recorded in multiple locations on Wednesday in the states of South Australia and Western Australia, as well as the Northern Territory.

The region's premier, Gladys Berejiklian, declared a state of emergency for the second time since last month, granting fire authorities additional powers to deal with the crisis. The Australian Defence Force was called on to assist firefighters.

Sydney's entire perimeter is threatened by blazes, and hundreds of houses have already been destroyed this fire season.

Ms Berejiklian said bushfires destroyed 40 homes on Thursday, with half being lost to the south west of Sydney in the Balmoral and Bargo region. She said the other 20 burned down in Buxton.

However the NSW Rural Fire Service later said conservative estimates suggested about 20 homes had been destroyed in total, with assessments still under way.

Ms Berejiklian said some of those south-west properties had belonged to firefighting volunteers. This brings the total number of homes destroyed in NSW this season to more than 800.

Three firefighters were taken to hospital with serious burn injuries after their crew was "enveloped by fire" near the Bargo area.

Two male firefighters, aged 36 and 56, were airlifted to hospital on Thursday afternoon with face and airway burns, as well as burns to their arms, elbow, upper chest and leg, Rural Fire Sevice Commissioner Shane Fitzsimmons said.

Helicopters have been collecting water from swimming pools to fight fires as dam levels reach critical levels across parts of the state.

Ms Berejiklian told ABC Sydney she was "absolutely confident" NSW has adequate resources to fight the fires.

"We've essentially pulled back everything we've given to other states," she said. "We've actually been planning for these conditions for some years.

"We'd assumed that things were going to be bad this summer and we took steps up to 12 months ago to make sure we were as well prepared as possible."

Ms Berejiklian said with so much bushland burnt out already, blazes were now moving closer to major towns and cities.

"While we've seen tragic circumstances where six people have died during the season already, major fire fronts weren't in this proximity to major population centres, and that's what we're dealing with now.

The premier stressed that she was "very concerned but people should remain confident the information they receive from authorities is accurate".

There is expected to be a lull on Friday before fire conditions peak again on Saturday.

As the fires rage, many Australians have been critical of the response of the conservative federal government and Prime Minister Scott Morrison — who is currently holidaying in Hawaii.

Green MP was among a group of protesters arrested outside of Mr Morrison's official residence in Sydney, Kirribilly House.