The fire danger ratings in Sydney, the Hunter and Southern Ranges has been upgraded from very high to severe.

Key points: Temperatures to reach 37C in Sydney and low 40s in the state's west

Temperatures to reach 37C in Sydney and low 40s in the state's west Total fire ban declared across four regions, from the Illawarra/Shoalhaven to the Hunter Valley

Total fire ban declared across four regions, from the Illawarra/Shoalhaven to the Hunter Valley Open fires, solid fuel barbecues, welders and grinders should not be used

The mercury in Sydney rose to 36 degrees Celsius around midday today and will reach a maximum of 37C, remain high overnight and then reach 30C on Saturday.

Similar temperatures were seen near Newcastle, the Central Coast and further south in Wollongong.

A total fire ban remained in place across parts of New South Wales today as temperatures are expected to hit 40C in the state's west.

The NSW Rural Fire Service (RFS) warned the combination of high temperatures and strong winds — gusting up to 60 kilometres per hour in some parts — could see fires break out quickly and easily.

"We're seeing temperatures in the high 30s and that is the first real heatwave of this season and probably one of the first … dangerous days… [with] all of the conditions coming together," Inspector Ben Shepherd said.

"Under these types of conditions fires can be erratic, they can move very, very quickly."

The fire bans stretch from the Illawarra/Shoalhaven and Southern Ranges in the south, up through Sydney and the Hunter Valley in the north.

They were raised from very high to severe around midday, with Mr Shephard saying the danger was upgraded due to drier conditions and lower humidity than expected.

Lifeguards on high alert

With the sweltering heat sweeping over the state, lifeguards at beaches and swimming pools are on high alert as people dive into the waters to cool down.

Surf Life Saving chief executive Steven Pearce warned Sydneysiders and beachgoers to be vigilant during heatwave conditions today.

"We're obviously are on high alert so all our lifesavers and our after call duty officers are on standby," Mr Pearce said.

People took advantage of the sweltering heat by swimming at Bondi Beach this morning. ( ABC News )

"Our lifeguards have been deployed to locations where we think it's going to have the greatest amount of patronage today on the beaches.

"Our helicopters are also on standby, which is a little bit strange this early out into the season."

Mr Pearce said Surf Life Saving have been preparing for the summer and have acquired drones to scout locations up and down the NSW coastline.