Sydney, the Hunter and the Illawarra will again face very high fire dangers on Wednesday as the mercury nudges 30 degrees and humidity levels dive.

The Harbour City is forecast to reach 28 degrees, as is much of the city, according to the Bureau of Meteorology.

The NSW Rural Fire Service said fire risks would be elevated by humidity levels dropping to as low as 13 per cent with north-westerly winds gusting to 55 km/h.

Sydney residents may again smell smoke on Wednesday morning from hazard-reduction burning in the Ku-ring-gai Chase National Park.

Air quality reached "poor" levels in Macquarie Park and Camden on Tuesday as smoke from the blazes settled over parts of the city.