The first large heatwave of summer has prompted a fire weather warning and a public health alert with only modest relief forecast for the next few days before the heat starts to build again.
Sydney's expected top temperature was trimmed by the Bureau of Meteorology to 35 degrees for the central business district - reversing an overnight revision higher.
Western suburbs were already reporting temperatures near or above 40 degrees by late morning..
The bureau earlier issued a fire weather warning for the Sydney metro and Illawarra/Shoalhaven fire areas.
Both districts have a total fire ban in place for the day.
"Very hot, dry and gusty northwesterly winds are expected ahead of a weak cold front that will bring a mild southerly change to southern and central coastal areas later in the evening," the bureau said.
Graeme Brittain, a meteorologist with Weatherzone, said sea breezes should start to pick up from about 1-2pm, bringing moderate relief to the CBD.
The line tempering of the heat, though, should only extend to about Strathfield.
"East of that will see cooling in the afternoon once the sea breezes come through," Mr Brittain said.
Bondi and other beaches may be the places to head, with that spot expecting a top of 30 degrees despite being only a few kilometres from Observatory Hill where Sydney's official readings are taken.
A cool changes of sorts - which is part of the prompt for the elevated fire risks - should reach Sydney at about midnight.
Much of Sydney can expect an uncomfortable night with a drop in temperatures towards the low-20s moderated by an increase in humidity, Mr Brittain said.
Sydney's CBD topped 30 degrees just after 10.30am, while Sydney Airport exceeded 35 degrees before noon.
Richmond and Penrith were vying for the hottest sites across the basin, with Penrith reaching 40.4 degrees just before midday to be the warmest in NSW. Richmond was about half a degree cooler..
Most of NSW is in for a sizzler on Thursday. (See Bureau chart below.)
NSW Health, meanwhile, has warned residents to try to stay cool and remain hydrated through the day.
"People can be unprepared for the first heat spike of summer, so we are reminding them to take safety measures against the effects of overheating and sun exposure," Ben Scalley, Director of Environmental Health, said in a statement.
"It's important people keep up their water intake, stay cool and avoid strenuous physical activity in the heat of the day," he said.
"Heat places a lot of strain on the body and can interfere with blood circulation and cause dehydration, heat exhaustion and heat stroke."
As reported by Fairfax Media, electric fans can provide considerable relief at as much as one-fiftieth the cost of air-conditioning.
Along with the heat, there is also a chance of thunderstorms in Sydney in the afternoon, as humidity starts to build.
"It's important people keep up their water intake, stay cool and avoid strenuous physical activity in the heat of the day."Ben Scalley, Director of Environmental Health
The mild change is predicted to bring the maximum to 26 degrees on Friday - slightly above Sydney's long-term December average of 25.2 degrees.
In places such as Penrith, the mercury is predicted to hit 42 degrees on Thursday and 32 degrees on Friday.
Sunday may be the day to get away a lot of the remaining Christmas shopping because more heat is on the way.
"The early part of next week could see another burst of heat for Sydney," Mr Brittain said, adding the heat will be widespread across Australia's south east.
According to the bureau's heatwave forecast, that warmth with include a fair chunk of inland NSW facing "extreme" conditions. (See bureau chart below.)
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