Just as the autumn chill was supposed to be biting, Shoalhaven has joined many parts of NSW in stringing together a record number of unseasonably hot days.

Nowra has gone well past the record for May, with 13 days this month above 20 degrees, easily surpassing the previous May record of 11 in 1974.

Bureau of Meteorology spokesman David Barlow said many parts of the state were experiencing maximum temperatures between two and four degrees above the average.

He said a slow-moving high-pressure system over the Tasman Sea was the cause.

"It's been sitting there, slowly moving eastward over the Tasman for the past week," he said.

The region’s regular swimmers know this time of year is beautiful in the water, and the coastal rock pools have stayed even warmer for longer.

But the high-pressure system has been the bane of the region's surfers, as its sheer breadth has caused the seas to stay calm for most of the week.

While Thursday and Friday will be warm, Mr Barlow said the temperature would cool somewhat by the weekend, with a shower possible on Saturday, then heating up again next week.