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Hotter than summer: Mercury soars in NSW heatwave

Sydney has sweltered through its hottest day in two years, as temperatures topped 40C in some parts and sent thousands streaming for the beaches.

A reprieve from high temperatures is not expected until Thursday, as fire authorities brace for days of scorching heat and warn of a high fire danger risk for much of the state.

The fire risk became reality for communities in the state's central west, with fires burning at a number of locations.

A blaze burned through 450 hectares and sparked an emergency warning for Toongi, south of Dubbo.

A similar call to shelter in place was issued for Tambaroora, south of Mudgee, as firefighters and aircraft worked to contain a 600ha blaze on Monday.

"Crews are working with the assistance of water-bombing aircraft and heavy plant machinery to contain the fire, which is burning in rugged and largely inaccessible terrain," the RFS said.

An uncontrolled 400ha grass fire was also burning in the Burrendong area, 15km southeast of Wellington.

The extremely hot and dry conditions blasted the mercury past Sydney's summer high of 30.6C, hitting 37.9C at 4.44pm.

It was also the hottest day since Australia Day 2021 to the west of the city at Penrith and Badgerys Creek where temperatures climbed past 40C.

While the temperature remained above 35C at 7pm in some parts, it was expected to ease overnight to about 21C before spiking again on Tuesday.

"It is not unusual to experience heatwaves during early autumn as the weather transitions from the warmer months of summer to the cold of winter," the Bureau of Meteorology said.

"The bureau's long-range forecast for autumn indicates it is likely to be drier and warmer than usual for much of Australia."

Meanwhile, thunderstorms - possibly severe, with damaging winds - may impact north-east NSW on Tuesday and Wednesday.

NSW Police deputy commissioner Peter Thurtell urged people to refrain from taking unnecessary risks in the extreme conditions.

"During extremely hot weather, we often see an increase in tragic incidents including drownings, falls from windows or balconies, and kids, pets or vulnerable people suffering distress or injury from being left in a hot car," he said.

Nearly 30 people drowned in NSW waterways during summer and emergency services are pleading for people to think before they swim.

"At the beach, always swim between the flags and listen to the directions and advice of surf lifesavers - please do not take a risk and swim at an unpatrolled area," Mr Thurtell said.

Elsewhere among the nation's capital cities, Brisbane and Perth are forecast to reach a maximum of 33C, while Darwin should approach 34C.

Milder conditions are expected in Canberra (25C) and Melbourne (22C), while Adelaide and Hobart are forecast to be cooler at 21C and 19C respectively.