It's the middle of winter but Thursday has felt closer to a summer's day in Sydney, as the mercury climbed 9 degrees above the July average of 16.4.

At a top of 25.2, it's easily the hottest day on record for the first week of July – a full degree above the previous record of 24.1 in 2004.

And it will remain relatively warm until a cool wind change on Friday afternoon, although even over the course of the weekend, temperatures will remain around 2 degrees above average.

The monthly temperature record for Sydney's Observatory Hill is 26.5, from July 30 last year.

If it reaches at least 23 tomorrow (the forecast is 24), it would be the earliest occurrence of two consecutive days above 23 degrees in July, and only the eighth time two such days have been recorded in July (since a Stevenson screen was installed at Observatory Hill in around 1910), according to weather bureau senior climatologist David Martin.