Sydney is having its warmest day in six months, although a blustery cool change is on its way up the NSW coast.

Warm northwesterly winds flowing across the Sydney Basin on Thursday afternoon caused the mercury to reach the low thirties in a number of suburbs.

As of 2pm, the temperature had reached as high as 30.9 degrees at Observatory Hill and 32.4 degrees at the airport. This is around 11-12 degrees above average for this time of year and the highest temperature since late March at both sites.

A cool and gusty southerly change moving up the NSW coast will march through Sydney later this afternoon between 3-5pm. The passage of the southerly buster will bring a rapid drop in temperature and a burst of strong winds, particularly near the coast. Gusts over 70km/h are a good chance for the coastal fringe just ahead of and with the wind change.

Looking ahead, wind will ease overnight and cooler air will stick around tomorrow. The city is only forecast to reach 20 degrees on Friday afternoon, more than 10 degrees cooler than it was today.