Video caption Australians on the heatwave: 'It's so hot, I feel like dying'

A state of emergency has been declared in New South Wales, Australia, amid fears a record-breaking heatwave will exacerbate the state's bushfire crisis.

The nation endured its hottest-ever day on Tuesday, but that record was smashed again on Wednesday - which saw an average maximum of 41.9C (107.4F).

Tuesday's 40.9C had eclipsed the previous record of 40.3C, set in 2013.

Authorities in New South Wales (NSW) are currently fighting about 100 fires, in a crisis that has lasted months.

Premier Gladys Berejiklian announced a seven-day state of emergency due to forecasts of worsening conditions.

"The biggest concern over the next few days is the unpredictability, with extreme wind conditions [and] extremely hot temperatures," she told reporters on Thursday.

Parts of NSW, of which Sydney is the capital, hit temperatures in the early 40s on Thursday. More intense heat was forecast for the rest of the week.

A state of emergency gives fire authorities additional powers to cope with the crisis. It is the second such declaration in NSW since last month.

The Bureau of Meteorology (Bom) said individual December temperature records had been broken in a number of places - with temperatures in the southern city of Adelaide hitting 45.3C.

Nullarbor, also in southern Australia, reached the scorching temperature of 49.9C.

Image copyright DANIEL KNOX/HORSLEY PARK RURAL FIRE BRIGADE Image caption Thousands of firefighters have been deployed to tackle the blazes

Bushfires have been raging in Australia for months, killing six people, destroying hundreds of homes and burning millions of hectares of land.

The crisis - worsened by tinder-dry conditions from a severe drought - has spurred criticism of the nation's climate policies.

A lot of outrage on social media has also been directed at Prime Minister Scott Morrison for going on an overseas holiday during the emergency.

According to local media, he is in Hawaii with his family. Some 500 people on Thursday protested outside his Sydney residence, demanding action on climate change.

In one widely shared tweet, the model who in 2006 became the face of the famous Australian tourism campaign slogan "Where the bloody hell are you?" directed the phrase at Mr Morrison.

Sorry, this Twitter post is currently unavailable.

What's behind the heatwave?

Australia heated up this week as a mass of hot air swept east across the continent.

The dominant climate driver behind the heat has been a positive Indian Ocean Dipole (IOD) - an event where sea surface temperatures are warmer in the western half of the ocean, cooler in the east.

The difference between the two temperatures is currently the strongest in 60 years.

Video caption Australia heatwave: All-time temperature record broken again

The warmer waters cause higher-than-average rains in the western Indian Ocean region, leading to flooding, and drier conditions across South East Asia and Australia.

Australian politicians have faced criticism recently for failing to link the bushfire crisis to climate change.

The science around climate change is complex - it is not the cause of bushfires, but scientists have long warned that a hotter, drier climate would contribute to Australia's fires becoming more frequent and more intense.

According to Bom, Australia has warmed overall by slightly more than 1C since 1910, with most of the heating occurring since 1950.

Officials predict that 2019, on the temperatures recorded so far, will be among the four warmest years on record.

Bom has urged people to keep cool, drink lots of water and follow the advice of emergency services amid the latest heatwave.

Temperatures are expected to exceed 45C in NSW, Victoria and South Australia.

Heatwaves are Australia's deadliest natural disaster and have killed thousands more people than bushfires or floods.

What is the heat doing to the fires?

Officials have warned that the combination of high temperatures and strong winds could heighten the crisis.

Two major fires are currently burning near the outskirts of Sydney, including a "mega blaze" which has burnt over 400,000 hectares.

Image copyright Getty Images Image caption Heavy smoke was seen blanketing the city's famous tourist sites

On Thursday, three firefighters suffered serious burns after being "enveloped by fire" near the town of Bargo, officials said. They added it was too early to say how many houses had been razed.

Smoke from the fires blanketed Sydney again, pushing air quality beyond "hazardous" levels in parts of the city.

A state-wide "total fire ban" has been put in place across NSW until midnight on Saturday.

Emergency services in the state of Queensland evacuated residents from dozens of homes on Wednesday as they battled to contain an out-of-control fire.

"You need to be ready to follow your bushfire survival plan. If you do not have a plan, or intend to leave, you should be ready to leave the area because the situation could get worse quickly," they warned.

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