Air quality worsened through parts of Sydney on Sunday, following hazard reduction burns to the city's west and north west.

Authorities had said the smog was likely to clear during the day, but at 11.30am, air quality remained at "hazardous" levels through much of the city's west.

Smoke across Sydney harbour on Saturday afternoon Photo: Nick Ryan

According to NSW Environment & Heritage figures, air quality had improved on Saturday night following earlier blanketing conditions, but deteriorated again on Sunday.

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The suburbs recording the worst air quality on Sunday morning were Prospect and St Marys in the city's west.

Rozelle, in the inner west, was recording the worst reading for inner-city suburbs earlier in the day. But by 11.30am conditions in Rozelle had improved, and air quality was worst in Earlwood to the south.

Smoke haze from a hazzard reduction burn hanging over Lake Illawarra in Wollongong on Saturday Photo: Adam McLean

The director of climate and atmospheric science at Environment and Heritage, Matthew Riley, said he expected stronger winds and rain on Sunday to help improve conditions throughout the day.

"The rain will help wash the smoke out of the air and the wind will help blow the smoke away," Mr Riley said.

Mr Riley said the poor air quality on Sunday morning was a result of the lingering effect of smoke from Saturday's hazard reduction burning, combined with new smoke from burn-offs on Sunday.

"There are still fairly high levels of smoke around," he said.

The NSW Rural Fire Service is reporting multiple hazard reduction operations throughout the state and greater Sydney. There are burn-offs taking place in the Blue Mountains, in the Hills District, and at Wollondilly. 

The Bureau of Meteorology has forecast a top of 23 degrees for the city on Sunday, with up to four millimetres of rain most likely later in the afternoon and evening.