Bureau of Meteorology forecaster Elli Blandford said the smoke was likely to hang around until Sunday when a wind change should bring some relief.
“We’re not really getting any wind directions that are going to be clearing out the smoke from the [Sydney] basin in the next couple of days,” Ms Blandford said.
Once pollution reaches hazardous levels, health authorities warn people to “significantly cut back on outdoor physical activities”.
Sensitive groups, including the elderly and those with asthma, should “avoid all outdoor physical activities”, the NSW Health website says.
“Everyone may experience more serious health effects. In Sydney, the [air quality] only reaches this level during major bushfires or dust storms.”
NSW Health director Richard Broome said the conditions meant people with chronic respiratory or heart conditions should avoid outdoor activity.
“If you really want to keep out of the smoke, the best thing you can do is stay indoors, keep your doors and windows shut,” Dr Broome said. “If you’ve got air conditioning, turn that on as well, which helps to clean the air inside.”
Residents in the Illawarra, the Central Coast and Hunter regions are also facing hazardous air quality.
Visibility at Sydney Airport was four kilometres, when it is usually more than 10 kilometres, the Bureau’s Ms Blandford said.
The Crestwood Drive fire at Lake Cathie has burnt 2845 hectares, with firefighters battling to contain the blaze before conditions worsen before the weekend.
On Thursday evening there were 70 fires still burning across the state, with 44 yet to be contained. Several areas of NSW are facing “very high” fire danger on Friday.
Sydney is forecast to hit a top of 26 degrees on Friday, while Penrith should reach 33 degrees. The air quality forecast is “poor”.
Josh Dye is a news reporter with The Sydney Morning Herald.