A heatwave is expected to turn Sydney's spring on its head this Saturday with temperatures forecast to soar past 36 degrees Celsius in parts of the city.

The Bureau of Meteorology (BoM) said Richmond and Penrith — in Sydney's west — would be over 30C for the next four days, peaking on Saturday.

Meanwhile, in western New South Wales, temperatures in the towns of Bourke and Brewarrina are forecast to hit 40C on Saturday.

Sydney's CBD is tipped to reach 33C on Saturday, while 29C is expected in Bondi.

Map Saturday's forecast temperatures

BoM duty forecaster Mohammed Nabi said the heatwave could break records.

"If we do get to 40C, it will be the first time that NSW has seen a 40 degree temperature in September," he said.

"Those temperatures are well above the September averages. We're talking in excess of 10 degrees above the average maximum for many parts of the state."

The NSW Rural Fire Service is expected to place total fire bans on some areas of the state as the heat sets in.

Bourke has seen it all before

Luke Moon, the owner of the Port of Bourke Hotel, said hot weather did not scare thirsty punters.

"It's hard to say until the day, but I can't see the temperature getting that high for this time of the year," he said.

The Port of Bourke Hotel is prepared for the heat. ( Supplied: Ally Newton )

"Out here it gets to 50 degrees in summer, so 40 doesn't scare us. That's what air con is for."

Mr Nabi said the heat in NSW would be caused by a clashing high pressure system in the Tasman Sea and a weak front over southern parts of the country.

"What happens is they start dragging very hot air from the central part of the continent down over NSW," he said.

"The main message is, it's really hot — even for summer.

"Try to stay out of direct sunlight. Stay in the shade if possible."