It will be a weekend of two seasons in Sydney, with a summer-like Saturday giving way to a mild and sunny Sunday.

Spring is usually a time of year when temperatures see-saw markedly in Sydney, as warm inland air is carried across to eastern NSW ahead of approaching cold fronts surging up from the south.

The mercury is forecast to reach 31 degrees across the Sydney Basin on Saturday, which would be the city's first day over 30 degrees this season and one of its warmest days this early in the season during the last decade. Since 2009, only two other years have featured 30 degree days this early in the season at Observatory Hill.

After a taste of summer on Saturday, a blustery change late in the day will drop the mercury across the Basin. Sunday is forecast to be more than 10 degrees cooler, reaching just 17 degrees in the city under sunny skies.

This weekend's warmth comes after Penrith hit 32 degrees on Wednesday afternoon, which was its highest temperature so early in the season for five years.

The average maximum temperature at Sydney's Observatory Hill during September is 20 degrees.