Record temperatures are possible across much of NSW on Saturday, elevating bushfire risks to "severe" in much of NSW, and increasing the health risks for those playing sport.
The NSW Rural Fire Service has issued total fire bans for 12 of the state's 21 districts on Saturday, including the Illawarra, Sydney and Newcastle areas. Similar restrictions may be extended into Sunday.
A region of "quite enhanced high pressure" over the interior of Australia had allowed heat to build up, Dean Sgarbossa, a senior forecaster on the Bureau of Meteorology's extreme weather desk, said.
A cold front skirting the continent's south is now pushing some of the unusual heat into the eastern states.
"There will be elevated fire dangers reaching up to extreme in parts of southern Queensland and northern NSW," Mr Sgarbossa said.
On Saturday, NSW may register its hottest September day, as well its first 40-degree day in the month, in records going back almost 160 years.
Australia, too, may have its hottest September day, the bureau said.
For the Illawarra, the forecast top of 33 degrees on Saturday would be about 13 degrees above the norm for September.
The heat comes just over a week after the city notched 33 degrees, the hottest day so early in the warming season.
Parts of the Illawarra should see the mercury reach 26 degrees by 9am on Saturday, at a time when many schools may be holding sports finals. Temperatures will most likely peak in the afternoon, during sport or other outdoor activities.
"Certainly, that would be the time to seek some shade, shelter and get some water," Ms Westcott said.
The worst of the heat will be in the state's north-west, with a large area expected to see temperatures close in on NSW's current September record maximum of 39.6 degrees set at Wanaaring in 2004.
Health NSW said it was monitoring the bureau's forecasts and is considering where to issue a heatwave health warning to the state's residents.
As the front moves eastwards on Saturday, gusty winds are expected to develop, worsening fire and other risks.
"We may see wind gusts in excess of 90km/h, so watch out for severe weather warnings - particularly in the eastern states," Andrea Peace, a senior bureau meteorologist, said in a statement.
Even after Saturday's heat peak passes, there's unlikely to be much of a reprieve for many in NSW with overnight temperatures to stay well above average, Ms Westcott said.
If Wollongong’s current forecast minimum of 20 degrees going into Sunday morning is accurate, it would be the warmest September night for the city since 2013, she said.
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