Traffic in Auckland's CBD is gridlocked after main roads and motorway onramps were blocked because of a huge fire at SkyCity Convention Centre.

New Zealand Transport Agency urged motorists to avoid the CBD or expect significant delays. As well as heavily congested and closed roads, bus services are also severely impacted.

Parts of Wellesley St, Nelson St and Hobson St are closed so fire engines can get close to the fire, which began about 1pm.

Multiple motorway ramps were also closed, including the Cook Street off-ramp, Fanshawe St off-ramp, and the Hobson St on-ramp to State Highway 1 and State Highway 16 before reopening.

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The Nelson St off-ramps into the central city were also closed temporarily.

AT staff have been deployed across the city to redirect commuters to temporary bus stops.

An AT spokesman said the main bus service affected is the Northern Express services.