Famous landmarks such as the Sydney Opera House and the Harbour Bridge disappeared behind the thick haze, which has been caused by ongoing bushfires on the city's outskirts. Experts have said climate change has left bushland tinder-dry.
Health officials are advising people to stay indoors and many who do go outside are wearing face masks.
Protesters gather at Sydney Town Hall, demanding the government do more to combat climate change.
"That's what climate science has told me and I completely agree with it".
"Resources are stretched, but they're being prioritised", he said.
Those with heart and lung issues have been told to avoid all outdoor physical activity, while all people in Sydney should limit their time outdoors.
A ferry sails on the harbor as thick smoke settles in Sydney, Australia, Tuesday, Dec. 10, 2019.
"Toxicity is very, very high", Costa told reporters.
RFS commissioner Shane Fitzsimmons says the southerly cold front could ramp up fires surrounding Sydney by dramatically changing wind direction. He described politicians' response as "pathetic". "Yesterday smoke was causing some of the worst air pollution in Sydney that we've ever seen - this is climate change".
"The national government has to provide leadership", Turnbull said.
"No one is listening, no one is doing anything". "I'm struggling breathing so much today".
"We've had silence, or worse still, distractions like the launch of the religious discrimination bill while we're facing this very real climate crisis".
Filter masks were in abundance during the rally.
Hospitals in the area have reported a 25% increase in the number of people visiting emergency rooms.
"However, people with conditions like asthma, emphysema and angina are at greater risk because the smoke can trigger their symptoms".
Smoke from the fires can be seen from space.
"But we now lack the operational tools to understand the extent of these impacts or to manage them".
Nearly 3,000 firefighters will be deployed to fight the more than 80 bushfires which continue to burn across the state, including a so-called mega-fire north-west of Sydney, which was formed when the huge Gospers Mountain blaze merged with the neighbouring Little L Complex, Paddock Run, Three Mile and Kerry Ridge fires.
One demonstrator mocks the "thoughts and prayers" approach to policy.
Three million hectares (7.4 million acres) of land has been burnt - the size of some small countries - and vast swathes of koala habitat scorched.
Authorities said they were concerned about a potentially severe bushfire season this year after Australia experienced its hottest summer on record.