NSW Premier Gladys Berejiklian has confirmed at least 40 homes have been destroyed on Sydney’s outskirts, and three firefighters seriously injured, as NSW’s bushfire crisis escalated dramatically on Thursday.

Ms Berejiklian said half of those homes were lost in the Balmoral and Bargo region, south-west of Picton, and the rest were razed in Buxton.

Some of those properties belonged to firefighting volunteers, she said.

The latest update brings the total number of homes destroyed by fire to more than 800 this fire season.

There are currently two emergency-level fires burning at Green Wattle Creek and Gospers Mountain.

Crews heading towards Buxton where police tell me the Green Wattle Fire is now threatening homes there @SBSNews #NSWfires #NSWbushfires pic.twitter.com/VbAAG3ByaT

— Charlotte Lam (@lottie_lam) December 19, 2019

Earlier, RFS commissioner Shane Fitzsimmons said he feared dozens of homes might have been lost at Balmoral.

He said it was too early to give firm numbers because of how fast the blaze was moving.

Three firefighters – two men aged 36 and 56 and a woman aged 28 – were taken to hospital after they were overrun by fire on Thursday afternoon.

Mr Fitzsimmons said the trio had suffered face and airway burns.

The two men were flown to hospital, while the woman was taken by road. Two more firefighters were treated at the scene.

“That fire front has been spreading very quickly and intensely in an easterly direction, east north-easterly direction under these west south-westerly winds,” he said.

“That is now being impacted by a southerly change, which makes for an increasing level of danger because you get increased volatility, erratic fire behaviour.

“It’s still a very difficult set of circumstances in and around the fire ground both for firefighters, emergency services, and also for the communities in and around that area being impacted.”

• For the latest bushfire information, visit the NSW RFS website

The moment flames of up to 50 meters tore through parts of Balmoral, south-west of Sydney @abcnews @abcsydney #NSWbushfires #NSWRFS pic.twitter.com/2SGFKBiPPi

— Mark Reddie (@ReddieNews) December 19, 2019

CAMPBELLTOWN TO MITTAGONG: Hume Mwy is closed btwn Narellan Rd & Mittagong due to a bushfire. Avoid the area & delay travel.

— Live Traffic NSW (@LiveTrafficNSW) December 19, 2019

Emergency warnings were also issued on Thursday afternoon for the huge Gospers Mountain fire that has burned for weeks north-west of Sydney, the Tianjara fire, near Sussex Inlet on the NSW south coast, and the Currowan Fire near Batemans Bay

The dangerous fires came as Premier Gladys Berejiklian declared a seven-day state of emergency across NSW – the second of this bushfire season.

Ms Berejiklian said NSW residents should reconsider their Christmas plans, as the state of emergency takes in Christmas Day.

“The decision to declare a state of emergency is not taken lightly. It normally only goes for seven days and you have to make sure conditions are serious enough for that declaration to be made,” she said.

Dangerous fire conditions are threatening large parts of the NSW, with more almost 100 bushfires raging and communities preparing for days of soaring temperatures.

Authorities have warned Sydney’s entire perimeter is threatened by blazes.

Of most concern is the huge Gospers Mountain fire. The 410,000-hectare blaze is burning out of control on multiple fronts in the Wollemi National Park and was at ‘watch and act’ level on Thursday morning.

A statewide total fire ban will remain in place across NSW until at least midnight on Saturday.

The Bureau of Meteorology had forecast temperatures in the mid 40s for most of inland NSW, while areas west of the Sydney CBD were expected to top 41 degrees.

Mr Fitzsimmons said Australian Defence Force aircraft, logistics and operations expertise would be put to use fighting fires.

Image: NSW RFS

Elsewhere, Adelaide and Canberra are also forecast to hit 40 degrees on Thursday.

South Australia will cop the brunt of Thursday’s heat, with the capital forecast to top out at 44 degrees.

Wudinna, in SA’s south-central region, is forecast to hit 49 degrees on Thursday, as is the Western Australian town of Forrest.

Bureau of Meteorology forecaster Simon Timcke said heat that had been building in the middle of the country was moving south, creating the extreme conditions.

The heatwave will continue to intensify in coming days. Southern and central Australia will experience temperatures up to 16 degrees above average by Friday.

Central areas of Queensland have been issued very high danger warnings for Thursday, as crews battle about 70 fires across the state.

Heading into the weekend, all coastal towns from central Queensland to the NSW border will have very high fire danger, with central and southern parts of the state expected to face severe danger.

Today will be very hot and smoky in many parts of the state. Make sure you follow these great tips from NSW to Beat the Heat: https://t.co/gKo1QhXlvP To check out the temperatures and latest forecast, go to the NSW page on the BoM website: https://t.co/CBv32iDpYL pic.twitter.com/pFyWCHgnPs

— Bureau of Meteorology, New South Wales (@BOM_NSW) December 18, 2019

Hottest day

On Tuesday, Australia sweltered through its hottest day on record, with the average temperature across the nation hitting 40.9 – breaking the mark of 40.3 set in January 2013.

In north-western NSW, the weather bureau’s Smithville outpost recorded 44 degrees on Wednesday afternoon – the highest in the state.

Other places also sweltered, with Wilcannia recording 42.5 degrees, Borrona Downs 43.1 and Tibooburra 42.9.

The heat has prompted health warnings from state authorities, with NSW Health medical adviser Dr Adi Vyas urging people to take extra precautions.

“We know that combined effects of bushfire smoke and extreme temperatures have potential to cause severe illness, hospital admissions and even death,” he said.

“Heat puts lot of strain on the body and can cause dehydration, heat stress and heat stroke. It can also make underlying conditions worse. People over the age of 75, people with chronic conditions and those who live alone are most vulnerable.”

Meanwhile, police have again warned drivers not to leave children in cars on hot days. Victorian paramedics said they were called to eight cars with children locked inside on Wednesday, when temperatures across the state reached 40 degrees.

Thursday is cooler for much of Victoria but there are total fire bans for the northern country and north-east districts.

There will be a statewide fire ban on Friday, when temperatures will soar again. In Melbourne, the mercury is expected to hit 43 degrees on Friday.

-with AAP