A woman has been seriously injured after she was flung from a vehicle when it crashed early this morning in the Waikato.

The car accident in Maramarua has left five people injured and downed power lines closing part of State Highway 2.

A police spokeswoman said it was believed the woman was not wearing a seatbelt at the time of the crash at four this morning.

Two people were seriously injured, including the woman, in the crash. One was flown to Middlemore Hospital.

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SH2 was closed between Maramarua and Mangatawhiri, but has since reopened.

Emergency services have left the scene.

Police said this morning's crash was a reminder of the importance of wearing a seatbelt.

Assistant commissioner, Road Policing David Cliff said by wearing a seatbelt chances of survival in a crash are increased by up to 50 per cent.

"Seatbelts save lives...it's that simple."

Meanwhile, a car has flipped on the Southern Motorway, southbound, after hitting a lamp post. Despite initial reports someone was trapped in the vehicle, a police spokeswoman said no-one was trapped and only one person received minor injuries.

Emergency services were at the scene, near the East Tamaki offramp.

Traffic is expected to build so the New Zealand Transport Agency is warning motorists to have patience.