Sydney woke up to a foggy start this morning with thick haze enveloping the city and causing ferry cancellations, flight delays and traffic difficulties.
Visibility was reduced to just 100 metres in some areas of Sydney, Weatherzone meteorologist Rob Sharpe said.
At Sydney Airport at 7am (9am NZT), it was down to 400 metres which was low enough to cause flight diversions.
International flights were diverted to Brisbane and Canberra, and had knock-on effects for domestic flights.
A Sydney Airport spokesperson told 2UE domestic flights were only mildly affected.
However, some passengers have reported sitting on the tarmac in Sydney and Melbourne for up to three hours.
All ferries travelling on Sydney Harbour and Parramatta River were cancelled for several hours and there were road crashes as motorists battled in poor visibility.
Sharpe said the combination of showers on Monday night, light winds and high humidity caused the unusual fog to form.
''The fact that the wind is still light in the Sydney area means that that fog is not breaking up,'' he said.
''It's going to require the sun to do some heating, and that will in turn increase the wind strength. It will be a very gradual thing this morning. We're not going to see this fog breaking up quickly at all.''
- Sydney Morning Herald