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The temperature in Sydney hit 45.8C at 2.55pm (AEDT) today, exceeding the previous Sydney record of 45.3C set on January 14, 1939.
The Bureau of Meteorology says the temperature is significantly higher than the 39C it had originally forecast because an expected breeze had not moved onto the coast from the sea.
"If the sea breeze moves inland it will cool off pretty quickly, so at the moment we're just watching to see what happens," meteorologist Julie Evans told AAP.
Sydney's weather is comparable with places in NSW's far west, such as Whitecliff, which have recorded temperatures around 44C on Friday.
But Sydney was topped by temperatures in Penrith, in western Sydney, which reached 46.5C and could even make 48C.
"It wouldn't surprise me if Penrith broke a record," she said.
As the state swelters, paramedics have been swamped with emergency calls.
As of 1.10pm, the Ambulance Service of NSW had responded to 21 cases of heat exposure, a third of which were for people over 60.
In addition, there had been 62 cases of unconsciousness or fainting, many attributable to the heat, as well as 22 instances of vomiting.
Ambulance crews were carrying extra bottles of water to distribute to patients, an Ambulance Service spokeswoman told AAP.
People have been reminded that extreme heat can easily lead to life-threatening situations if precautions are not taken, such as staying indoors, drinking lots of water, and staying cool.
Elderly, sick and very young people were at greater risk of heatstroke, the Ambulance Service said in a statement.
Sydney's extreme heat is forecast to ease this evening when a cool change will sweep through the city accompanied by strong winds.
"Because it is so hot it will be very gusty when the change comes through," Ms Evans said.
While Sydney swelters, the state record - 49.7C at Menindee in 10 January 1939 - looks like it will stand firm.
"Out west we don't have temperatures anywhere near that, so Menindee's record is safe - but Sydney's isn't," Ms Evans said.