The state ambulance service reported an increase in calls from people suffering breathing problems, and officials urged people with asthma or heart or lung diseases not to go outside.

"Keeping yourself indoors today is the main thing to do if you have any of those conditions and particularly if you're a known sensitive sufferer such as children, older adults or pregnant women," said Wayne Smith, a senior state health official.

Sydneysiders coughed and hacked their way through their morning commute, rubbing grit from their eyes.