A teenager was thrown over rocks and seriously injured after falling from a quad bike on a beach this morning.

The 17 year-old suffered a head injury after falling at around 10am on a South Island beach near Mangarakau, west of Collingwood. He was airlifted to hospital.

The Nelson Marlborough Rescue Helicopter Trust said the teenager was riding an "off road quad-style bike with friends" when he lost control and was thrown several metres over rocks.

"The helicopter landed on scene where the male was assessed and initially treated by the on board Intensive Care Paramedic," the trust said.

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He was taken to Wellington Hospital's emergency department in a serious but stable condition.