Where did winter go?

That's the question many Sydneysiders are likely to be asking on Friday, with more temperature records set to be challenged in the city and across NSW during the day.

The mercury climbed to levels more typical of December than mid-winter, with the Bureau of Meteorology lifting its forecast top for the Harbour City to 25 degrees.

At its warmest, the temperature reached 24.8 degrees, just shy of Thursday's 25.2-degrees top.

Sydney has never posted two consecutive days of 25 degrees in July in 160 years of records at the site.