Thick smoke from nearby wildfires has blanketed Sydney in hazardous smoke, prompting experts to warn those with medical conditions to remain indoors.

The skyline of Australia‘s most populous city was barely visible on Tuesday, as officials said the air quality was measured at 10 times over the level considered hazardous.

Bushfires in Australia have killed at least four people this month, destroyed more than 300 homes and burnt around 2.5 million acres of farmland and bush.

Powerful winds fanned around 130 fires, which have been burning across New South Wales and Queensland states for several days, pushing smoke south over Sydney.

One resident of the harbour city said it was “choking in smoke” and another tweeted: “Woke up at 5am because my brain was trying to tell me I was in danger. I could smell smoke. I can still taste smoke. The fire is 100 miles away. We can barely see 100 metres in Sydney today.”

Sydney covered in haze from bushfire smoke Show all 8 1 /8 Sydney covered in haze from bushfire smoke Sydney covered in haze from bushfire smoke Smoke haze covers Sydney as wildfires burn near the city AAP Image/Reuters Sydney covered in haze from bushfire smoke The annual Australian fire season, which peaks during the Southern Hemisphere summer, has started early after an unusually warm and dry winter AP Sydney covered in haze from bushfire smoke Sydney Opera House and Harbour Bridge in the background as a man takes a picture Getty Sydney covered in haze from bushfire smoke Smoke haze covers Sydney, Tuesday, Nov. 19, 2019, as wildfires burn near the city. The annual Australian fire season, which peaks during the Southern Hemisphere summer, has started early after an unusually warm and dry winter. (AP Photo/Rick Rycroft) Rick Rycroft AP Sydney covered in haze from bushfire smoke Smoke from bushfires blanket Sydney on November 19, 2019. - Sydney woke up to a thick blanket of smoke as New South Wales warns residents of the dangers amid severe fire dangers and hot, windy weather. Bushfire-prone Australia has experienced a horrific start to its fire season, which scientists say is beginning earlier and becoming more extreme as a result of climate change, which is raising temperatures and sapping moisture from the environment. (Photo by PETER PARKS / AFP) (Photo by PETER PARKS/AFP via Getty Images) PETER PARKS AFP via Getty Sydney covered in haze from bushfire smoke SYDNEY, AUSTRALIA - NOVEMBER 19: A man takes a picture at Mrs Macquarie's Chair in front of a smoke covered Sydney skyline on November 19, 2019 in Sydney, Australia. NSW remains under severe or very high fire danger warnings as more than 50 fires continue to burn across the state. Six lives and 530 homes have been lost since NSW bushfire season hit, with more than 420 homes destroyed in the past fortnight alone. (Photo by Mark Kolbe/Getty Images) Mark Kolbe Getty Sydney covered in haze from bushfire smoke epa08007129 The Sydney Opera House seen through smoke haze in Sydney, Australia, 19 November 2019. Sydney has awoken to a thick blanket of smoke as New South Wales residents are urged to 'stay vigilant' amid severe fire dangers and a hot, windy weather forecast. EPA/JOEL CARRETT AUSTRALIA AND NEW ZEALAND OUT JOEL CARRETT EPA Sydney covered in haze from bushfire smoke SYDNEY, AUSTRALIA - NOVEMBER 19: Smoke shrouds the Sydney Opera House on November 19, 2019 in Sydney, Australia. NSW remains under severe or very high fire danger warnings as more than 50 fires continue to burn across the state. Six lives and 530 homes have been lost since NSW bushfire season hit, with more than 420 homes destroyed in the past fortnight alone. (Photo by Cassie Trotter/Getty Images) Cassie Trotter Getty

Officials warned people to stay indoors as much as possible as the smoke is likely to linger over the coming days.

“We know that heatwaves cause severe illness, hospital admission and even deaths, and that people are more sensitive to heatwaves early in the season,” Richard Broom, director of environmental health at NSW Health said.

“The combination of heat and poor air quality adds to the risk.”

In NSW, firefighters were scrambling to strengthen fire containment lines ahead of forecast higher temperatures for much of the rest of the week.

“More than 1,300 firefighters are working on these fires, undertaking backburning operations and strengthening containment lines ahead of forecast hot, dry and windy weather, with seven areas under a total fire ban,” the NSW Rural Fire Service said in a statement.

The current bushfire crisis has mostly been contained to the east coast of NSW and Queensland states, but officials in South Australia warned on Tuesday that forecast near-record temperatures raises the risks in that state.

Australia’s Bureau of Meteorology said temperatures in Adelaide, the capital of South Australia, will hit 42 degrees Celsius (107.6 degrees Fahrenheit) on Wednesday, which coupled with strong winds will create “catastrophic” fire danger conditions.