The Bureau of Meteorology (BOM) is urging Sydneysiders to "check up on their elderly neighbours" on Sunday, with the mercury expected to reach 45 degrees Celsius in some parts of the city.

Several areas in Sydney's west are predicted to swelter through the weekend, with highs of 45C in Penrith, 44C in Richmond and 43C in Liverpool, Blacktown and Campbelltown.

The CBD and eastern suburbs will not be immune, with the bureau warning temperatures in those areas could top 40C.

"Heatwaves can particularly affect vulnerable people, so the sick, the elderly and children," BOM duty forecaster Rob Taggart said.

"People should check up on their elderly neighbours, stay in cool conditions, keep hydrated and make sure that you're looking after those who might be vulnerable."

While the heatwave is not expected to break records, it could extend for several days in some areas of the state.

"Sea breezes should keep conditions fairly cool on Saturday," he said.

"On Sunday that really hot air will come right through to the eastern suburbs as well."

NSW Rural Fire Service inspector Ben Sheppard said total fire bans had not been imposed at this stage.

"It is still going to be risky," he said.

"It is still going to be very hot and there are some light winds around, but also that's going to be tempered by humidities that are a little bit higher."

Big power users in New South Wales are also expecting to be asked to cut back over the weekend, as people switch on their air conditioners.

Bluscope Steel, in the Illawarra, was last year asked to reduce its demand for power during heatwaves and is likely to be approached again this year.

"We will be doing what we can, but we have technical limitations on our usage," a spokesman said.

Bluescope cannot power off completely because of the danger of damaging the plant.