Thick fog is still blanketing Sydney, but is expected to burn off over the next hour.


Visibility was reduced to 20 metres across parts of central Sydney, and banks of fog extended across large parts of coastal NSW and Canberra.


Ferry services in Sydney Harbour are suspended, except for the Manly Ferry, and foghorns could be heard from 7am as ships negotiated the harbour in a grey cloud.


There are current delays on the Sydney Harbour Bridge, while traffic was also delayed on the Great Western Highway and M2 and M4 due to hazard reduction burning in the Blue Mountains, AAP reported.


Motorists were warned to drive slowly, stay alert, and remember to use their headlights.


The Sydney Basin saw its most widespread thick fog of the year, as a high pressure ridge combined with light winds and high moisture levels, meteorological service Weatherzone said.


Once the fog clears, Sydney can expect a stunning Mother's Day, with generally clear skies and a top of 23 degrees, three above average for May.


Canberra will start off beautiful with mostly sunny skies, before cloud moves in from the afternoon. Despite the cloud, it should remain dry throughout the day with a top temperature of 22 degrees, six above average for this time of year, Weatherzone said.