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NSW swelters through second scorcher as bushfires burn

Houses are under threat from an out-of-control bushfire expanding on two fronts in NSW's central west as the state swelters through an autumn heatwave.

The blaze in Tambaroora, north of Bathurst, was upgraded to an emergency-level warning as it expanded across its northern and southeastern flanks.

"Go inside and protect yourself," the Rural Fire Service told residents.

"Do not be caught in the open in the path of the fire."

Evacuation centres were set up in Mudgee and Bathurst as fire crews used water-bombing aircraft to tackle the blaze, which had burned through an area three times the size of Sydney Airport.

Another 40 blazes, 26 of them uncontrolled, were burning across the state on Tuesday afternoon.

Temperatures in Sydney topped 34C in the CBD after reaching 38C on Monday - the city's hottest day in two years.

At Penrith, in the city's west, the mercury hit 36.9C, while Williamtown, north of Newcastle, recorded a high of 37.9C.

Easterly winds provided some relief for coastal areas but the mercury in Campbelltown, Richmond and Badgerys Creek hadn't dipped below 34C by 6pm.

Wednesday will be more bearable for most across the state.

The Bureau of Meteorology cancelled its heatwave warning, with average or below-average daytime temperatures expected in most centres.

The northeast can still expect higher temperatures.

The central ranges, Hunter and northwestern regions will all be subject to total fire bans, with high danger in several other areas.

RFS Commissioner Rob Rogers said the fire load had increased dramatically in the past few years during extremely wet La Nina conditions.

"It's a very different environment than we were at going into that 2019/20 fire season when we were in the back of a four-year drought and there was basically no grass anywhere," he said.

"The good thing is we're not into drought but obviously the bad thing is all that grass fuel and that's going to be around for a couple of years."

Thousands of Sydneysiders were left to bake without electricity on Monday night as supplier Ausgrid scrambled to restore power to 11,000 customers.

The company said the outages were the result of a number of isolated incidents that damaged infrastructure, not the heat.

"We acknowledge it was a really bad time to be without any power or cooling," an Ausgrid spokeswoman said.

NSW set a new March record for electricity demand as residents battled the evening heat, according to the Australian Energy Market Operator.

The state's energy operational demand was 13,136 megawatts of power, eclipsing the previous record from March 2016.

The bureau said it was unusual to experience heatwaves during early autumn, while forecaster Sarah Scully said cooler air was on its way.

"The west, northwesterly winds are notorious for bringing really hot conditions over eastern NSW and that's because the air is brought from over the inland continent and it's really hot and generally dry," she said.