Sydneysiders set for heat reprieve

After an unseasonable run of sweltering spring weather, relief is in sight for Sydneysiders with a cool change expected on Wednesday.

Temperatures soared into the high 30s over the long weekend, lodging some of the warmest on record for the start of October.

Penrith topped 38C on Tuesday, well above Sydney's October average of 22C.

But the weather bureau says Wednesday will bring near-average conditions for this time of year, with maximum temperatures set to drop between 10 and 15 degrees in some areas.

"Tomorrow will be a significantly cooler day for Sydney," forecaster Chris Webb told AAP.

"We're expecting low 20s, which is average, and even over the southern inland where it's particularly hot."

The reprieve will take a little longer to reach northwest NSW, and the town of Bourke is expecting a Wednesday high of 38C.

Ninety per cent of the state remains on high fire danger alert with 270 NSW firefighters and 102 vehicles attending calls on Tuesday.

There are 39 bush, grass and scrub fires burning across the state, of which 19 remain uncontained, a Rural Fire Service spokesman said.

He said none of them posed a direct threat to homes but people should remain aware of local fires.

Bureau of Meteorology senior climatologist Agata Imielska said the "warm spell" is the first of many to come.

"The El Nino will be with us until the end of summer and with that, we're expecting warmer daytime temperatures and an increased number of heat waves for spring and summer," she said.

Showers are possible this weekend as the change brings humid air across the city.