A nine-year-old boy was airlifted to Starship Hospital after a boat propeller cut his leg to the bone this afternoon.

The boy was struck by the craft at full speed at Tinopai in Northland about 4pm, a St John Ambulance spokesman said.

"It was quite a deep cut and the boat was going at full speed,'' he said. "The bone has been exposed".

Local fire fighters gave the boy first aid until ambulance and a Northland Emergency Services Trust helicopter arrived to transport the boy to Starship Hospital in Auckland.