Sydney hits 30C after long cool spell

Sydney marked its hottest temperature in 331 days on Wednesday as the mercury in many parts of the city soared past 30C.

Maximum temperatures had stayed below 30C since February 21 last year - the longest cool streak since a 339-day record set in 1883.

The run ended when Sydney's Observatory Hill recorded 30.2C on Wednesday, officially breaking the long run of sub-30C days.

"At around 2.30pm it did break that 30C ... that record run has ended," Bureau of Meteorology spokesman Patch Clapp told AAP.

In Sydney's west it was even hotter, with Penrith hitting 35.1C, while Richmond logged 34.9C.

Mr Clapp said hot temperatures had been kept in check until recently by easterly winds, as well as more rain and cloud cover across the city.

A rare third consecutive annual La Nina was also a factor, he said, with the weather event known for producing wet and windy summers.

The national forecaster earlier this week said that while La Nina persisted, it had weakened from its peak, predicting a return to neutral conditions in February.

Mr Clapp said Sydney's heat would not last long, with a cold change set to drop the mercury to the mid-20's on Thursday.

"We're getting this front coming across the state ... that's approaching towards Sydney," Mr Clapp said.

BoM, on its website, forecast possible morning showers for Sydney on Thursday and the chance of a thunderstorm.