A helicopter has crashed at speed into a paddock near Te Awamutu, injuring a passenger.

The pilot of the privately owned Safari 400 had been joyriding with a friend when the engine began "over speeding", police said.

Te Awamutu constable Ryan Fleming said the helicopter had just cleared power lines and was 10 metres high when the engine malfunctioned.

"He's realised straight away he was in trouble and he's tried to auto rotate which is land the chopper when there's no power but there was too much forward air speed and they were coming in too quick."

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Witness Grant Organ said the helicopter bounced before coming to a stop.

The scene of the crash on farmland south of Hamilton. Photo / Supplied
The scene of the crash on farmland south of Hamilton. Photo / Supplied

Smoke began pouring out and he raced over to the scene from the Mangapiko Hall where he had been working about 10.25am.

"The young boy crawled out screaming, 'My back, my back'. Then he just flopped down."
Mr Organ said the pilot, in his 50s, who is understood to build helicopters, walked away unscathed from the crash.

A second witness, painter Chris Campbell, called emergency services and the injured man, thought to be in his late teens or early 20s, was taken by ambulance to Waikato Hospital with moderate injuries.

The pilot also went to hospital.

Mr Fleming said it was too early to say what caused the crash and Civil Aviation would now determine whether to investigate.