Air quality across parts of NSW has reached hazardous levels and people have been warned to take care outdoors, as a large dust storm engulfs the coast.

Five NSW regions including Sydney's north-west, the Central Coast and the upper and lower Hunter regions recorded hazardous air quality levels - and the storm is forecast to get worse before it gets better.

"I’d expect this to thicken up in Sydney over the next few hours, probably peaking in the middle of the day, early afternoon," Bureau of Meteorology forecaster Anita Pyne said.

"It's also going to be quite windy, so potentially quite unpleasant in Sydney this afternoon."