The NSW Rural Fire Service has issued total fire bans for 12 of the state's 21 districts on Saturday, including the Sydney, Newcastle and Illawarra areas. Similar restrictions may be extended into Sunday.

For Sydney, the forecast top of 33 degrees for both Saturday and Sunday would be about 13 degrees above the norm for September.

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.

The heat comes just over a week after Sydney notched 33.8 degrees, the hottest day so early in the warming season, and not far shy of the record high for a September day of 34.6 degrees set in 1965.

Western suburbs may set a few records, with Bankstown predicted to reach 37 degrees, a forecast more than a degree hotter than its current September record of 35.6 degrees set in 1980, Kim Westcott, a meteorologist with Weatherzone, said.

Penrith's predicted 37-degree maximum could also eclipse the current record at that site.

Many parts of Sydney should see the mercury reach 30 degrees by 11am on Saturday, at a time when many schools may be holding sports finals. Temperatures will most likely peak about three hours later, 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.

Fire threat intensifies

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.

Ben Shepherd, an RFS spokesman, said many areas would experience high temperatures, low humidity and strong wind gusts on Saturday.

"This would be a bad day any other day", but given the dry conditions in most of the state, fire risks will be heightened, he said.

In Richmond, to Sydney's northwest, a top of 37 degrees is forecast along with 9 per cent humidity and wind speeds of 35-55 per cent. In Katoomba, in the Blue Mountains, the bureau predicts a top of 29 degrees and winds could reach 80 km/h.

Similar combinations are expected along much of the coast, while the northern Riverina may see some of the strongest wind gusts to accompany the heat.

For Hay and Griffiths, for instance, the bureau is predicting tops of 37 degrees for Saturday with 8 per cent humidity and wind gusts that could reach 100 km/h, Inspector Shepherd said.

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 Sydney's current forecast minimum of 21 degrees going into Sunday morning is accurate, it would be the warmest September night for the city since the record mild September night in 2003. Then the mercury dipped only as low as 22 degrees.

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