Traffic is heavy on Auckland's motorways this morning, with surface flooding following an early morning dump of rain and at least three crashes around the city.

The NZ Transport Agency warned airport-bound drivers to allow extra time with two separate crashes on the Southwestern Motorway heading south.

A crash that happened about 6.45am blocked the right lane on Mangere Bridge, with drivers warned to merge carefully in the wet conditions.

Further south a crash just before 6.30am blocked the left lane southbound after Lambie Drive.

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Both crashes have been cleared but traffic is heavy from Hillsborough to Onehunga Wharf.

On the Southern Motorway a crash just before 7am blocked the left lane citybound where the South Eastern Highway merges with the motorway at Penrose.

The crash has been cleared from lanes but traffic is heavy from Highbrook, and again in the usual areas from Drury through to Manurewa.

On the Northern Motorway traffic is heavy from Oteha Valley Rd through to Esmonde Rd and moderate through Onewa Rd.

The Northwestern Motorway is heavy from Hobsonville Rd through to Royal Rd and again from Great North Rd through to the CBD.