A teenage boy has been flown to Waikato Hospital with a broken arm after falling at least eight metres down a cliff and landing on his head while mountain biking.

The Auckland Westpac Rescue Helicopter rushed to the scene on a trail near Whangamata just before 1pm today.

A male in his teens had fallen eight to 10 metres down a cliff, landing on his head and back at the bottom.

He had a broken arm along with various cuts and abrasions following the fall.

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Auckland Westpac Rescue Helicopter crewman Aaron Knight said the boy was "very lucky".

"It could have been a lot worse," he said.

The boy was flown to Waikato Hospital in a moderate condition.