Air quality across the city improved on Tuesday morning though it was still rated "very poor" in the city's southwest and "poor" in the eastern suburbs at 9.30am.

A layer of air in the atmosphere is trapping smoke - meaning the haze will take time to clear in some areas, the Bureau of Meteorology says.

"Chances are it could still be pretty hazy (on Tuesday)," duty forecaster Steph Spackman told AAP.

"We won't have a clear air change until later in the week associated with the rain and the front that will be coming through."

Authorities have warned older adults, children and people with heart or lung disease to avoid outdoor exercise when air quality is very poor.

The NSW Rural Fire Service will on Tuesday ignite a new fire at Menai and maintain burns in the Wollondilly, Hawkesbury and Royal National Park areas.

Further south, a series of fires are expected to be started in the Kosciuszko National Park that will eventually burn across 3000 hectares.

"The blocks that we are burning are identified as part of an ongoing strategic three-year planning process and are located to minimise the run of wildfires in summer," NSW National Parks and Wildlife Service Branch Director Mick Pettitt said in a statement.

Australian Associated Press