Sydney residents coughed and spluttered their way around Australia's largest metropolis Thursday, as a bank of smoke from rural bushfires enveloped the city, prompting health warnings.

Smoke from out-of-control fires to the north of the city prompted health authorities to warn the city's residents with respiratory problems to avoid outdoor physical activity.

"If you really want to keep out of the smoke, the best thing you can do is stay indoors, keep your doors and windows shut," said Richard Broome of the New South Wales ministry of health.

Firefighters are struggling to contain dozens of blazes, including one caused by a lightning strike far north of the city that has ravaged an area of over 2,000 hectares (4,942 acres).

Strong winds have pushed the fumes toward Sydney, shrouding landmark buildings like the Opera House and Harbour Bridge and lacing the air with an acrid taste.

"The smoke will likely persist for several days and may not clear until some stage over the weekend," said New South Wales Rural Fire Service said Thursday.

In contrast to many of Asia's major cities, Sydney's residents and visitors usually enjoy the city's beaches and woodland under crystalline skies and pleasant sea breezes.

Authorities on Thursday rated the air quality as "hazardous", with a higher concentration of particles per million than cities like Bangkok, Jakarta or Hong Kong.

Rare koalas feared dead in bushfire

Meanwhile, hundreds of koalas are feared to have burned to death in an out-of-control bushfire on Australia's east coast, wildlife authorities said Wednesday.

A bushfire believed to have been sparked by a lightning strike on Saturday some 400 kilometers north of Sydney has ravaged an area of over 2,000 hectares (4,942 acres) with authorities battling to bring it under control.

Wildlife rescuers in northern New South Wales state hold grave fears for a "very rare" population of hundreds of koalas living in the fire zone.

"The special importance of those koalas is that they are very genetically diverse," Sue Ashton, president of the Port Macquarie Koala Hospital, told AFP.

Her fears are that "hundreds" in the known koala breeding area "have perished in the fire".

"It's a national tragedy because this koala population is so unique," she added.

Land clearing and development over time has meant a loss of habitat for the tree-dwelling koalas, leading to less connectivity between populations, increased inbreeding and reduced genetic diversity of the marsupial.

More than 70 fires continue to burn across the state of New South Wales, which has been battling severe drought.

Authorities say conditions are easing near Port Macquarie, where large air tankers are being used to battle the out-of-control blaze.

"The fire will continue to burn throughout the night, however there are significant resources in the area protecting properties," New South Wales Rural Fire Service said in their latest update.

Ashton said wildlife volunteers would join firefighters in the area either Thursday or Friday to assess the scale of loss and begin a rescue operation for the surviving koalas.

"What happens to a koala in a fire is that they climb up to the top of the tree and they curl up into a little ball. If the fire goes through quickly and just singes their fur, they are fine the fur will grow back," she said.

But if the fire intensifies and continues to burn up the tree "they'll perish", she added.

"So we won't really know until we get in there and start to look."