Sydney's east had an air quality index of 407, while the highest reading recorded in the north-west was 130, which is in the "poor" range.

Camden and Campbelltown West had "very poor" readings of particulates of 2.5 microns in size - small enough to enter the bloodstream - over the past day.

"The smoke could hang around quite a bit today - this is normal, it can happen when there's lots of hazard reduction," said Stephen Stefanac, a meteorologist at the Bureau of Meteorology.

"Air cools during the night, which keeps the smoke lower to the ground. When it heats up in the afternoon, the smoke should clear."

During what is known as a temperature inversion, the warmer air is held above cooler air - which is the opposite of what normally occurs, with air usually getting colder at a higher altitude.

Smoke resulting from hazard burns at ground level had nowhere to go, resulting in a layer of smoke in large parts of NSW.

In terms of public safety, Mr Stefanac said to follow NSW Department of Health warnings and to monitor the air quality index.

As of 6.30am, dozens of scheduled burns were being carried out across the state, according to the Rural Fire Service's map.

An RFS spokesman said the smoke around Sydney was from three large burns that were started on Sunday.

He said the burns at Sutherland, a 5500-hectare burn in the Bargo area south-west of Sydney, and burns at Colo Heights in the Hawkesbury region would continue throughout Monday.

"We will experience smoke throughout the day today and it's likely that we will experience it again tomorrow morning," the spokesman said.

The spokesman said most of the work would be completed on Monday but the fires could still be active on Tuesday.

He said there were also a few smaller burns scheduled for the Southern Highlands during the week.