Updated December 14, 2017 11:05:30

Photo: Parts of the harbour city are expecting temperatures above 40C. (Graeme Powell, file photo: ABC)

Much of NSW will swelter through its first heatwave of summer today, with temperatures in Sydney's west expected to soar above 40 degrees, but the city's east should be spared thanks to a sea breeze.

Penrith is forecast to reach 43C today, while the city will reach 35 degrees.

NSW Health is urging people to stay cool and hydrated to prevent heat-related illnesses.

"People can be unprepared for the first heat spike of summer, so we are reminding them to take safety measures against the effects of overheating and sun exposure," director of environmental health Dr Ben Scalley said.

Bureau of Meteorology duty forecaster Rob Taggart said very hot air was sitting over inland NSW yesterday, with temperatures reaching 43 degrees, and now that air was coming over western Sydney.

"[But] the places near the coastal Sydney areas, they'll be spared because sea breezes will come in and prevent that really hot air from coming in to the eastern suburbs," he said.

A total fire ban has been declared for the Greater Sydney and Illawarra-Shoalhaven regions.

Mr Taggart said a cool change would come through Sydney about midnight offering some relief tomorrow, but the rest of the state would not be so lucky.

"The northern inland parts of the state will remain very hot over the coming seven days with temperatures in the high 30s or even the 40s ... most of the time," he said.

"The north-west and the central-west [will be] particularly affected with temperatures increasing up into the mid-40s early next week and Tuesday in particular looks to be the hottest for western districts."

Stay inside and check on elderly relatives

Heatwaves kill far more people than other natural disasters. ABC Emergency has a checklist of things you can do to be ready.

Dr Ben Scalley from NSW Health advised people to stay inside, drink lots of fluid and check on their elderly relatives and friends.

He said dehydration and heat stroke were common health problems during heatwaves, but there were also increased incidents of other illnesses.

"Heat has a lot of effects on the body in terms of stressing the body and we even see increases in effects like heart attacks during these periods," he said.

"It's really important that people take precautions to protect their own health."

Topics: weather, sydney-2000

First posted December 14, 2017 06:46:13