Astonishing pictures show Sydney blanketed in thick smoke as bushfires continue to rage in Australia.
Still weather conditions trapped the haze over the city as the air quality index (AQI) reached 12 times the level deemed hazardous.
It is the worst case of the problem to hit the city since Australia's crisis began several weeks ago.
Dr Richard Broome, from the New South Wales (NSW) Health department, said Sydney was suffering "some of the worst air quality we've seen".
"The smoke here in Sydney is extremely bad today. We are urging people to take this seriously," Dr Broome told the Australian Broadcasting Corporation.
With visibility reduced in some areas to as little as 500 metres, all Sydney Harbour ferries were cancelled on Tuesday, with extra buses deployed to transport regular ferry users.
Transport officials said late on Tuesday afternoon the ferry service had been reopened on one major route - between the city and the northern beachside suburb of Manly - but that all other routes remained closed.
The AQI in the inner suburb of Rozelle was record as 2,552 on Tuesday afternoon. Any region with an AQI of more than 200 is considered "hazardous".
The NSW Rural Fire Service said 85 fires were burning across the state, 42 of them uncontained.
Many Sydneysiders took to wearing facemasks to stop inhaling the smoke, although health authorities have issued warnings to ensure they used effective kinds of masks and that they were worn properly.
"The evidence is that whilst they can be effective, they might not be providing the benefits that people think they are," Dr Broome said.
And as people were urged to stay indoors, several facilities were quick on the uptake to invite people in, including the State Library of NSW.
Former Oasis singer Liam Gallagher, who is visiting the city, tweeted: "Sydney looks spooky".
The Australian Council of Trade Unions (ACTU) cautioned that people should not be working outdoors unless it was unavoidable.
ACTU Secretary Sally McManus tweeted: "Here is some context to today's highest Air Quality Index reading of 2100 in Sydney. The city coming in second is New Delhi on 670. No one should be working outside unless you are emergency services."
