Thick fog is shrouding Auckland this morning causing traffic to move at snail's pace on the city's roads.

Fog is blanketing western and northern parts of the city with virtually zero visibility along sections of the Northwestern Motorway.

The Upper Harbour Highway and top of the Northern Motorway are also cloaked in fog as rush-hour traffic moves slowly through the trying and hazardous conditions.

The NZ Transport Agency is advising drivers to using fog-affected motorways to turn on lights and drive carefully.

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Other parts of the city are enjoying clear, if slightly grey, conditions.