First, the good news — the smoke that has blanketed parts of Sydney for large chunks of this week is dissipating in some areas.

Now for the bad news. It's about to come back.

Key points: Sydney's air quality has been rated "hazardous" for three days this week

Sydney's air quality has been rated "hazardous" for three days this week Much of the smoke has come from fires burning in northern NSW

Much of the smoke has come from fires burning in northern NSW They are not likely to be extinguished for weeks, which means the smoke will stay

The Harbour City's air quality plummeted this week after a combination of smoke from dozens of bushfires, and dust from drought-stricken western New South Wales settled in the Sydney basin.

On Tuesday and Thursday, the Air Quality Index (AQI) at most locations in Greater Sydney was rated "hazardous".

This morning, most locations around the state had an AQI over 200, which means they are "hazardous".

LOCATION AQI Sydney CBD 229 Richmond 651 Liverpool 300 Wollongong 207 Newcastle 245 Wagga Wagga 308

Smoke that had settled over Sydney's CBD and eastern suburbs was blown further west by a southerly wind change this morning.

A view of the smoke on Thursday, as seen from the Sydney Tower. ( AAP: Dylan Coker )

Bureau of Meteorology (BOM) forecaster Abrar Shabren said that haze, and a large plume of smoke that has settled offshore, means the smoke will persist over the weekend.

"We are surrounded by smoke all across the greater Sydney area, Illawarra and further north," he said.

"There will be thick smoke in and out of the west today.

"The smoke will drift back across the CBD and east tomorrow overnight."

The smoke was particularly bad over Granville, in Sydney's west, on Thursday. ( Twitter: @swopnil42 )

Much of the smoke has been blown over Sydney from bushfires burning in the state's north.

The NSW Rural Fire Service (RFS) has been fighting some of those blazes for months, and there is no end in sight.

That means the smoke will also likely continue to be a problem in Sydney for the foreseeable future.

The difference in Sydney's air quality is plain to see around the Anzac Bridge. ( Twitter: @LaPetitTrader )

RFS spokesperson Anthony Bradstreet 1.6 million hectares of land in NSW had already been burnt.

"There's about 6,000 kilometres of fire edge. It really will take a reasonable rain event to extinguish them," he said.

"Firefighters are working on trying to contain the fires.

"Without rain, they could burn for weeks and months."

The smoke was so bad yesterday morning that several locations had an AQI over 1,000, with Prospect reaching 1,699 and Parramatta North 1,239.