Drivers were being urged to take it easy on the roads as a heavy fog covered Sydney on Monday morning.

Ten domestic flights were cancelled and one international flight was diverted as Sydney residents woke up to a thick blanket of fog, reducing visibility to 50 metres in some parts of the greater metropolitan area.

The worst affected area was the south-west, where visibility was down to 50 metres at Camden.

Visibility dropped to 100 metres in Campbelltown in the south-west and Richmond in the north-west.