NSW Health has warned that wearing masks "might not be providing the benefits that people think they are" as Sydney's air quality index (AQI) skyrocketed due to bushfires on the city's outskirts.

Key points: Dangerous smoke is choking almost all areas around Greater Sydney

Dangerous smoke is choking almost all areas around Greater Sydney Many believe they have never before experienced such poor air quality in Sydney

Many believe they have never before experienced such poor air quality in Sydney The Bureau of Meteorology is predicting the smoke to linger all week

The AQI in some areas reached 12 times hazardous levels on Tuesday, as the city was blanketed in thick smoke.

Dr Richard Broome, from NSW Health, said Sydney was experiencing "some of the worst air quality we've seen".

"The smoke here in Sydney is extremely bad [Tuesday]. We are urging people to take this seriously," Dr Broome said.

Dr Broome issued a warning to Sydneysiders wearing masks to protect themselves from the smoke.

"P2 or N95 face masks are in theory effective against smoke, but there is evidence that in order for them to work, you have to have a really good fit and it is quite hard for people to achieve that in practice," Dr Broome warned.

"So the evidence is that whilst they can be effective, they might not be providing the benefits that people think they are."

Pedestrians are seen wearing masks as smoke haze from bushfires. ( AAP Images: Steve Saphore )

The worst air quality reading was in Rozelle on Tuesday afternoon, where the AQI was 2,552.

However, the smoke appears to be clearing with AQI numbers across Sydney dropping. Rozelle's rating was later recorded to be 473.

Any region with an AQI over 200 is considered "hazardous" — here are some of the hot spots as of 4pm:

Region AQI Camden 1,055 Liverpool 1,055 Richmond 454 Rozelle 497 Randwick 497

All ferry services have resumed after being cancelled for most of the day due to the thick smoke.

Numerous buildings across Sydney were evacuated because of smoke alarms, including the the Rural Fire Service (RFS) headquarters in Homebush.

Parts of the state's Central Tablelands, north west, Hunter, Illawarra and south west regions are also experiencing dangerous levels of smoke.

Dr Broome said this level of air pollution means everyone, regardless of their health, should cut back on outdoor physical activity.

"The people who are most vulnerable are people who have existing heart and lung problems," he said.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 37 seconds 37 s Sydney Harbour barely visible through smoke

NSW Health recommended people stay indoors between 11.00am and 4.00pm. ( AAP Image: Joel Carrett )

While smoke has been near-permanent fixture in Sydney this bushfire season, people took to social media on Tuesday to lament the "worst" air quality they had seen.

"Woke up [Tuesday] morning to the worst smoke haze I have ever seen in our local area," Samantha Waterfield in Belmont North said.

"Definitely the worst the conditions on the Northern Beaches have been. I've had no voice for three days from the smoke, I can't imagine how people living closer are coping," Kelly Owens said on Twitter.

A southerly change brought cooler conditions to firegrounds in NSW on Tuesday afternoon, with temperatures dropping as much as 10 degrees Celsius in a matter of minutes, but it has also brought increased winds.

Earlier, temperatures in Penrith in Sydney's west had topped 40 degrees but the RFS said it would be some hours before challenges lessened for firefighters.

NSW Rural Fire Service Commissioner Shane Fitzsimmons said it would be a "very complex, very difficult day.