Sydney and parts of New South Wales have sweltered through an "unseasonably warm" weekend, with temperatures climbing more than 10C above the September average, before a cool change swept much of the coast on Monday. This was reported by Qazaqyia.kz citing The Guardian.

The early burst of heat, part of a classic "spring rollercoaster", brought summery hot weather conditions to the state.

Weatherzone reported that a wave of heat swept across south-eastern Australia over the weekend, before the peak of the heat hit Sydney on Monday.

Temperatures in Sydney's outer west reached 35.4C in Richmond and 35C in Penrith on Monday, just before a forecasted cold snap via a south-south-easterly change.

The city also sweltered. Sydney Observatory Hill reached a high of 32.7C at 10.40am, before temperatures plummeted significantly by about 10C within a couple of hours, a Bureau of Meteorology spokesperson told Guardian Australia on Monday afternoon.

On the weekend, easterly sea breezes brought some relief to coastal areas, keeping eastern Sydney a few degrees cooler than the west. On Sunday, western suburbs recorded maximums of about 34C, while the city recorded 28.9C.

The spokesperson said temperatures were "unseasonably warm in many parts of the state", generally sitting 6C to 12C above the September average.

"It was quite warm compared to the September average in Sydney, say, by more than 10C," they said, noting the average September maximum for Observatory Hill is 20.1C, and 23.5C for Penrith.

The hot, dry and windy conditions ahead of the southerly change prompted authorities to issue high to extreme fire danger warnings for Monday, particularly for the Greater Hunter, much of the north-eastern parts of the state, and northern inland areas.

For now, Sydneysiders can now expect a much cooler, wetter outlook for the coming days under the influence of a cool southerly airstream.

"We expect the temperatures to be on the cooler side for tomorrow into Wednesday ... together with cloud overcast and light showers," the bureau spokesperson said.

On Tuesday, Sydney city temperatures will hover in the high teens, between 18C – 19C, before slightly warming up to between 22C and 24C on Wednesday. Temperatures will then gradually rise through the rest of the week, before another cooler change arrives next weekend.

Regional centres including Cessnock, Muswellbrook and Singleton were forecast to reach 35C amid extreme fire risk, according to Weatherzone.

However, the bureau said fire danger "will abate for much of the state" from Tuesday, in the wake of the cool change arriving.

A strong El Niño developing in the Pacific Ocean is forecast to peak in late spring or summer and "reach levels above the warmest observed since reliable records began in 1950", according to the BoM.

Climate experts have said El Niño will probably combine with climate change to push global temperatures to record highs in 2027. This winter was Australia's third-warmest on record and the hottest on record for New South Wales, Tasmania and Victoria.

In Australia, El Niño events increase the chances of warm and dry weather, upping fire danger in summer – especially in regions that haven't had much rain recently.

Authorities told NSW residents to refer to the state's rural fire service (RFS) website for the most up-to-date information regarding local fire risks and warnings.