Hazard reductions to stop ahead of heatwave

Hazard reduction burns have been suspended for 48 hours to relieve the haze of smoke that has choked Sydney for the past four days

Sydney's air quality has been affected since burnoffs started on Sunday, with it rated as one of the five worst cities in the world for air quality yesterday morning.

Fortunately, the haze is expected to pass by Sunday as windy conditions blow the smoke away from the city.

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But at the same time, firefighters are warning that an incoming heatwave poses a huge bushfire risk for the city.

Temperatures are expected to reach the high 30s across Sydney next week, while strong wind gusts are also expected.

Sydney's central east, south-west, and north-west regions have been particularly afflicted this week, with the air quality dropping to "poor or worse" this morning, according to the Department of the Environment.

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People are urged to again keep doors and windows closed and spend limited time outside, particularly if they're physically exerting themselves.

But the runners preparing to take on the Sydney Marathon on Sunday have been told the air should clear by then.

Police have said 42,000 people have registered to take part.

The full marathon will see runners travel from Milsons Point to the Sydney Opera House forecourt.

Recent shots of Sydney Harbour and the bridge in the mornings have shown heavy smoke in the area.

The major burns, however, are now wrapping up, the Rural Fire Service says.