A motorcyclist was airlifted to hospital with serious injuries after being thrown from his bike on a beach south of Auckland.

The man was catapulted from his bike after striking a hard mound of sand on Saturday while travelling along Karioitahi Beach, about 70km southwest of downtown Auckland.

Surf Life Saving Karioitahi head of lifesaving Mike Lawrence said lifeguards found no external injuries but he was in a confused mental state.

"Further secondary assessment revealed possible chest and abdominal injury," he said.

The man was airlifted by the Westpac rescue helicopter to Middlemore Hospital with serious head injuries and broken ribs.