Two truck and trailer units have collided with each other on Mangakahia Road leaving one driver seriously injured, police say.

Two vehicles - one heavy truck and trailer unit and one light truck and trailer unit - have been travelling north on Mangakahia Road, about 33km out of Whangarei, when the heavy truck has crashed into the back of the light truck about 2.15pm, police said.

The driver in the front vehicle is uninjured and the other driver has been seriously injured but is in a stable condition. He is being flown to Whangarei Hospital.

Police said the Serious Crash unit is in attendance and the road is open for light vehicles.

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The public is asked to avoid the area for the next hour or so while debris from the road is cleaned up.