One of the first people on the scene of a truck and motorbike crash on Auckland's Southern Motorway this morning has described the motorcyclist's horrific injuries.

Ben Fisher was driving from Karaka to an appointment in the city when he heard a bang and saw someone fly into the air.

After getting out of his car he saw what appeared to be a woman with a severely gashed leg.

"I have never seen the inside of a leg before," Fisher told the Herald.

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He said he ran down the motorway looking for a first aid kit and got one from a concrete truck driver.

The motorcyclist's injuries were too severe to treat, said Fisher, who said he and a couple of other people put a blanket over the woman. The motorcylist lost consciousness before the emergency services arrived 5 to 10 minutes later.

"It was pretty ghastly," said Fisher.

He did not see the crash and did not know what caused it, but believed motorcyclists needed to take more care.

"I've seen it time and again. Motorcyclists passing cars, weaving in and out, and this is what happens. They should go with the flow and not weave in and out of the traffic," said Fisher, who owns a paint contracting company.

Fisher also helped get one lane open during the morning rush hour until the police arrived.

One person was seriously injured in the crash, which happened near the Panama Rd overbridge in Mt Wellington.

Police said two lanes initially blocked by the crash have now been cleared but the NZ Transport Agency warned drivers to allow extra time for congestion - or consider an alternative route.

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