Libraries, council chambers and art galleries across NSW could be used as air-conditioned "chill-out areas" during future heatwaves under a plan by Labor.

The state opposition, which first pitched the idea in 2017, is urging the NSW government to trial opening some public buildings after-hours as respite cooling centres for the elderly and families.

“Unfortunately, there are families unable to turn on their air conditioners due to high electricity costs or those who simply cannot afford air conditioning,” Labor leader Michael Daley said in a statement on Thursday.

“The heat can also be devastating for the elderly.”

He said Labor would trial the plan if elected at the March poll.

The Bureau of Meteorology has forecast four consecutive days above 40C across much of NSW, with extreme conditions to peak on Friday.