The man was taken to hospital with serious injuries to his face.
A Facebook user later made a post in a West Auckland group saying the dog had been running along the beach when "a young lad of about 10 ran across the path of the dog and was knocked over".
The user said the boy was helped to his feet and as the dog owner was putting the lead on his pet, the boy's "cowardly" father kicked him in the face and left him unconscious in the water.
Kakamatua Inlet is considered one of the best dog beaches in Auckland.