Smoke from rural bushfires is seen over Sydney Harbour on Thursday. Saeed Khan/AFP

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

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Smoke from out-of-control fires to the north of the city prompted health authorities to warn Sydneysiders 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.

A jogger is seen in Blackwattle Bay as smoke haze drifts over the CBD in Sydney, Australia, on Thursday. Reuters

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 Kong.

Agence France-Presse