Drew Hall says he was fishing with his parents in a dinghy off the north coast of New South Wales state on Sunday morning when the whale breached nearby.
He said Monday he did not see the tail coming when it crashed down on the 17-foot (5.2-meter) dinghy, smashing its canopy.
His mother Karen Hall said she saw her son knocked to the floor and thought he had been killed.
The boy was taken to a hospital for treatment of a concussion and a broken collar bone. He was discharged Sunday afternoon.