Parts of NSW just had their warmest July day in more than half a century.

A stream of air flowing over NSW from central Australia caused temperatures to climb five to nine degrees above average across most of the state on Thursday.

Gunnedah (24.8C) had its warmest July day in 60 years, while Goulburn (19.7C) and Wagga Wagga (20.1C) registered their highest July temperatures in 43 and 42 years, respectively.

Tamworth (23.4C), Trangie (23.4C) and Dubbo (22.9C) all had their warmest July day in more than two decades.

Further east, Sydney registered its warmest day on record for the first week of July, reaching a top of 25.3 degrees in the city shortly before 3pm. This beat the previous record of 24.1 degrees from the first week of July in 2004.

The warm air will linger over northern, central and eastern NSW into Friday, although a cooler southwesterly change will flush the heat out of the state by the weekend.