Sydney will be spared a third consecutive day of searing temperatures on Sunday, but the rest of the state won't be so lucky as the fire danger intensifies to a catastrophic level.

The mercury rose above 40C in 55 regional towns on Saturday, with Ivanhoe Airport recording a blistering 47.6 just before 4pm.

Sydney's CBD reached 36.2C as the city's west sweltered through Saturday, with Richmond recording 47C and Penrith 46.9C.

While temperatures aren't even tipped to reach a balmy 30C in Sydney on Sunday, much of NSW will remain above 40, as fire dangers reach unprecedented levels.

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"To put it simply, they're simply off the old conventional scale," Rural Fire Service Commissioner Shane Fitzsimmons said on Saturday.

"It's not another summer day, it's not another bad fire weather day. This is as bad as it gets."

Citing Australia's worst bushfire disaster, the Black Saturday blazes which claimed 173 lives in February 2009, Mr Fitzsimmon said "the forecast indices (for Sunday) are greater than those faced by Victoria at the time".

However "at this stage no fires are posing a threat", Mr Fitzsimmons told Sky News on Saturday.

"They are all at advice level and we are doing a lot of work."

The monster hot air mass hovering over NSW had had "a baking effect" on vegetation, which meant extreme warnings would pose major risk on Sunday.

Conditions were set to intensify in centres including Dubbo, Coonabarabran and Narrabri in the north through to the Hunter Valley and the coast at Port Stephens.

Mr Fitzsimmons urged people to prepare early and stay out of danger by spending time at the the movies or a local shopping centre.

The Bureau of Meteorology said very hot conditions associated with the mass of air and combined and fresh westerly winds over the southern half of the state meant severe fire danger throughout Saturday for the Central and Southern ranges, Lower Central West and Hunter districts.

The Australian Market Energy Operator said NSW faced a possible lack of power supply on Saturday afternoon, with the risk increasing into the evening.

AGL said it would cut back on electricity use at its Tomago Aluminium Smelter in Newcastle, which uses about 10 per cent of the state's power.