One person has been taken to hospital in a serious condition after a crash that forced the closure of part of Auckland's busy Karangahape Rd.

A St John spokesman said one person had been taken to Auckland Hospital in a serious condition and another person was being assessed for moderate injuries. The crash involved a scooter and a car.

The westbound lanes of Karangahape Rd between Pitt St and Howe St are closed.

Auckland Transport is advising motorists to avoid the area or follow the directions of emergency services.

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Meanwhile, a large truck is blocking part of the Northern Motorway heading into downtown Auckland and motorists are being told to find another route.

Transport authorities said about 12.30pm the truck was blocking the left lane, citybound, of the State Highway 1 Northern Motorway near the Silverdale on-ramp.

The NZ Transport Agency said: "Avoid this route or merge with care and expect delays.''

Images from NZTA cameras show long lines of vehicles backing up towards Silverdale.

Earlier today a crash on the Southern Motorway resulted in delays in and around the Manukau area.

The accident happened about 11.45am near the southbound lane just before the Orams Rd overbridge.

Authorities cleared the area just after midday, but motorists were told to allow for extra time due to congestion.