Breathing masks are selling out in Sydney with the city enveloped in the smoke from bush fires sweeping across a large swath of Australia's east coast, damaging the country's clean and green reputation.

Office workers wearing protective masks, previously a rarity, have become a common sight in recent weeks in downtown Sydney, where record pollution levels have consistently ranked the city above the likes of Jakarta, Shanghai and Mumbai.

The world-famous Opera House and Harbour Bridge have regularly been shrouded by thick smoke that has turned the daytime sky a dark orange. Ash has fallen like a shroud over the sky, propelled by strong winds from the fires burning on the outskirts of greater Sydney.