The Bureau of Meteorology said it took about an hour for the temperature to drop from 32 to 26 at Sydney Airport at 1am on Wednesday, while the cool change didn't hit Penrith until 4am, when the mercury gradually fell from 30 to 21.

Temperature readings from Sydney's Observatory Hill last night.

People on the south coast, where temperatures were in the low 30s very early on Tuesday morning, woke to a much cooler day on Wednesday.

The cool change hit Nowra at 10pm on Tuesday, dragging the temperature from 33 to 24 in just 20 minutes. It was 16 degrees at 7.30am.

"It's a very significant drop, a very quick one," Bureau of Meteorology duty forecaster Dmitriy Danchuk said.

"This change was pushing temperatures down quickly all along the coast.

"In Sydney it took a little bit longer because of a pool of hot air, and the change was struggling a little bit."

Mr Danchuk said a cold front moving across the Tasman Sea, linked to a low pressure system southwest of New Zealand, had brought cool air to parts of NSW.

"The cold air started moving along the coast and effectively spread to the southern and southwestern parts of the state," he said.

The change was moving gradually up the coast and would slow down when it hit the northern rivers area, Mr Danchuk said.

"It's struggling against high temperatures.

"It moved across a good half of the state during the night, but we're still expecting pretty high temperatures [in the north] because it will be moving very slowly."

The system had most recently brought cool air to Williamtown and Newcastle, on the coast, and Dubbo and Bourke, in the west.

But the cool weather won't last long, with the bureau predicting Sydney to gradually warm up again on Thursday.

"On Friday and Saturday we'll see temperatures soaring to low 40s in the west, but the coast could see temperatures in the mid-30s," Mr Danchuk said.

"It will be very warm to hot conditions again, not that hot like yesterday, but it will be another burst of hot temperatures."