The boy was being towed in a ski biscuit with another boy when the accident occurred about 3pm. He was flown to Randwick Hospital with serious head injuries.

Police said two ski boats were travelling in opposite directions on the river. One was towing a wakeboarder, while the two boys were being towed in a ski biscuit behind the other boat.

Police said it was believed the two boats nearly collided head on, however one of the boats took evasive action to avoid the collision and came into contact with the ski biscuit.

Both children were thrown into the water, one suffering a significant head injury, while the other suffered minor injuries and was treated at the scene.

Police took statements from the drivers - a 24-year-old man from Pyree and a 47-year-old man from Kiama.

Police have asked anyone who saw the collision and has not yet spoken to police to contact Shoalhaven Police via Crime Stoppers on 1800 333 000.