Sydney and Brisbane were shrouded in a spectacular fog on Friday morning, causing disruption to airports and ferries, and leaving landmarks barely visible.

The Sydney Harbour Bridge almost disappeared in the white blanket of fog as visibility was reduced to 200 metres in some parts of the city, according to the Bureau of Meteorology.

Two Sydney-bound international flights were diverted to Melbourne, while a Qantas flight from Los Angeles due to land in Brisbane was diverted to Noumea, local news agencies reported.

Ferries in Sydney were running as normal but services were suspended in Brisbane due to low visibility on the Brisbane River.

Despite the foggy start, Brisbane was forecast to experience a warm winter's day on Friday, with temperatures expected to reach 25 degrees Celsius.

Sydney was also expecting a comfortable high of 20 degrees Celsius.