Four bushfires flared to emergency level, at least 20 properties have been destroyed and three firefighters have sustained serious injuries as temperatures climbed past 42 degrees Celsius in Sydney.

Key points: Gusty conditions are blowing heat and smoke across Greater Sydney

Gusty conditions are blowing heat and smoke across Greater Sydney NSW Health has issued warnings, with the mercury hitting 42C in some areas

NSW Health has issued warnings, with the mercury hitting 42C in some areas Australia's hottest day ever was Tuesday, but the record stood for only 24 hours

Greater Sydney endured a dangerous day of heat and fires, with temperatures climbing as high as 42.4C at the airport as flames surrounded the city.

At 1:00am, three fires were still burning at emergency warning level.

The Green Wattle Creek fire was burning out of control at Wollondilly in Sydney's south-west, with properties under threat, while the Kerry Ridge fire and Gospers Mountain "mega-fire" were burning north of the Blue Mountains.

There is a concern that the Kerry Ridge fire could merge in the coming days with another fire burning to the south at Paddock Run.

The Currowan fire, on the South Coast, had been at emergency level earlier on Thursday but was downgraded to watch and act late on Thursday night.

NSW Rural Fire Service (RFS) Commissioner Shane Fitzsimmons said firefighters battling a blaze at Balmoral, south west of the Harbour City, had been seriously injured after being "overrun" by a blaze.

A 36-year-old and a 56-year-old were airlifted to hospital with serious face, arm and chest injuries, while a 28-year-old woman was transported by ambulance to hospital.

Live footage from an ABC helicopter showed several homes destroyed by fires while RFS crews worked to defend other houses in the area.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 38 seconds 38 s Firefighters have been injured battling the blaze at Balmoral

NSW Premier Gladys Berejiklian said the blaze at Balmoral had destroyed at least 20 properties on Thursday.

A "poor air quality" alert has been issued by the NSW Government as the thick plumes of smoke from surrounding bushfires descend on Sydney.

Much of the smoke was caused by a so-called "mega-fire" burning at Gospers Mountain, which has already torn through about 420,000 hectares of land.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 1 second 1 m 1 s Firefighters battle extreme conditions as Green Wattle Creek fire flares ( Mark Reddie, ABC News )

Camden, Campbelltown West and Liverpool in Sydney's south-west recorded an Air Quality Index (AQI) of 673 on Thursday afternoon.

Anything over 200 is considered "hazardous" for health.

Sydney's CBD reached a high of 39.3C while the mercury in Penrith and Parramatta peaked at 42.5C.

Other hotspots were Gosford (42.1C), Albion Park (43.4C) and Condobolin Airport 42.3C.

Firefighters have been battling wild blazes at Bilpin, in the Hawkesbury area. ( ABC News: Brendan Esposito )

Ms Berejiklian declared a state of emergency across NSW for the next seven days from Thursday.

"The biggest concern for us over the next few days is the unpredictability," the Premier said.

"With extreme wind conditions, extreme hot temperatures, we have a good idea, a good sense, of where the most concerning areas are, but again when you've got those turbulent conditions, embers and spot fires can occur very unpredictably."

It is the second state of emergency the NSW Government has declared since the start of the bushfire season.

The south-western edge of the Gospers Mountain 'mega-fire' glows as it burns towards Springvale, near Lithgow. ( ABC News: Jonathan Hair )

Commissioner Fitzsimmons said 2,000 firefighters had already been deployed but another 500 had been put on standby due to the extreme conditions.

"The forecast is that Saturday will be even worse," he said.

"We're talking about some very serious and dangerous conditions over the coming days"

Gusty winds from the north-west have blown a heatwave that is gripping central Australia to the east coast.

The Bureau of Meteorology (BOM)'s Neil Bennett said much of the country had been affected by a "dome" of hot air.

Firefighters have been suffering under the strain of the unrelenting bushfires, with at least three seriously injured. ( ABC News: Brendan Esposito )

Mr Bennett said Friday would bring some relief, but Saturday would heat up again, with temperatures in the mid-40s throughout the state.

The BOM said Australia recorded its hottest day on record on Tuesday, with an average maximum temperature of 40.9C.

But the record was smashed on Wednesday, when the average maximum temperature from around the country was 41.9C, a whole degree over the record set a day earlier.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 53 seconds 1 m 53 s NSW Premier Gladys Berejiklian has declared a state of emergency across NSW.

SafeWork NSW executive director Andrew Gavrielatos said the heatwave brought significant risks to the health and safety of the public.

"Many people simply don't notice the symptoms of heat stress and by the time they are seriously impacted the ability to self-assess is compromised," he said.

"Employers must take heat stress seriously. Consult with your workers and take action to ensure workers are not exposed to heat stress."