NSW has broken the record for the highest September temperature for the second time in a week.
The mercury reached 40.8C in Bourke in the state's northwest on Wednesday, making it the hottest September day on record in NSW.
It beat the previous record of 40.5C set last Saturday in Wanaaring.
"We have broken the all-time state record for maximum temperatures in September twice in a week," Bureau of Meteorology senior meteorologist Andrew Haigh told AAP.
"That's something that has not happened before."
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While Wednesday's temperatures didn't reach as high as predicted, most of the state's upper western district reached 38 degrees.
The record-breaking temperatures this September reflect an "extreme" weather pattern, Mr Haigh says.
The mercury will dip for most of the state on Thursday with the exception of the far north coast, which is predicted to reach more than 35C.
A total fire ban has been issued for the far north coast on Thursday.
