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A Fire and Emergency New Zealand spokesman said it was notified at 6.30am about a truck driving south of Milton on State Highway 1 going off the road into a ditch.
Crews and appliances from Milton responded initially.
The driver was trapped underneath his vehicle, but was conscious and able to talk to rescue crews.
Extra heavy duty rescue equipment had to be brought in from Dunedin and appliances from Dunedin Central and Lookout Point stations attended.
The driver was freed at 8.24am.
A St John spokeswoman said one ambulance and one rescue helicopter attended the scene.
The man was in a serious condition, she said.
A police spokeswoman said one lane is still blocked.