Temperatures in Sydney's west are expected to hit 39 degrees Celsius before thunderstorms bring rain possibly by the late afternoon.
People in Penrith and Richmond are expected to swelter on Wednesday as temperatures soar to levels eight degrees above the summer average.
In central Sydney and Wollongong, the mercury is tipped to reach a maximum of 32 degrees Celsius, which is six degrees above average.
Bureau of Meteorology duty forecaster Peter Zmijewski said desert winds were to blame for the soaring temperatures across NSW.
"North, northwest winds are bringing these hot temperatures," he told AAP.
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He said there was a 30 per cent chance there would be thunderstorms during the late afternoon or early evening.
"There's a chance of showers and thunderstorms," Mr Zmijewski said.
"It's very hard to put a timing on it. I would say late afternoon, evening."
A high fire danger remains in place all down the NSW coast, along with the northern tablelands, and the Riverina and Murray River areas in the state's south.
A very high fire danger is in place for the Illawarra.
