Heatwave conditions have engulfed NSW with temperatures exceeding 30 degrees soon after the sun came up.

With a forecasted 43 degrees for Sydney, the third highest temperature on record, Bega on the NSW south coast was the first town to hit 40 degrees at 11.30am (AEDT) on Tuesday.

It recorded 37.8 degrees at 9am after the temperatures rose almost 15 degrees in an hour, up from 23 degrees at 8am.

Temperatures of 40 degrees were recorded in Sydney at 12.30pm, up from 27 degrees at 9am.

Senior meteorologist at the Bureau of Meteorology (BoM), Julie Evans, says Sydney temperatures have trailed the rest of the state.

"Sydney had a late start, so to speak, because of mid-level clouds," she said.

"In this sort of situation, the temperature can jump a couple a degrees in a few hours.

"The upper level temperatures are really warm, and when it's windy, those high level temperatures mix down to the surface."

On Tuesday morning, temperatures along NSW's south coast increased by two degrees every 30 minutes, starting at 7am.

Sydney is expected to peak at 43 degrees in the afternoon, with the temperatures hovering around the 30 degree mark for most of the evening.


"We are expecting the change to reach Sydney and Newcastle around 4am tomorrow."

According to the BoM's automatic weather station, the hottest place in NSW is just west of Kangaroo Valley, with a temperature of 42.7 degrees.