A child has been flown to hospital after being struck by a boat propeller at Pakatoa Island in the Hauraki Gulf this afternoon.

The child, a boy, was flown to Starship Children's Hospital in a serious condition, Auckland Rescue Helicopter Trust communications' manager Lincoln Davies said.

The alarm was raised at 4.10pm and the crew of Auckland Westpac Rescue Helicopter's Westpac 2 was called to help the boy.