The shark swam away when the boy's father jumped into the water, but the child suffered lacerations across his body, Ambulance Tasmania said on Friday.
He is in stable condition following the incident, which took place off Stanley on Tasmania's northwest coast.
"The boy, from the North-West, was aboard a six-metre vessel on a fishing expedition about five kilometres from shore with his father and two other men when a shark grabbed him from the boat," authorities said.
"The boy's father jumped into the water at which point the shark swam off," they added. "The boy, who was wearing a personal flotation device, suffered lacerations to his arm, and other cuts to his chest and head."
The attack is the latest in a number of shark attacks in Australia so far this winter.
On Tuesday, Anika Craney, 29, was attacked while swimming off Fitzroy Island near Cairns, CNN affiliate Nine News reported. She was on her day off from filming a documentary about sharks when she was attacked, and is now recovering in hospital.
A 15-year-old boy died in a suspected shark attack while surfing in New South Wales, police said on Saturday, Australia's fifth fatal shark attack this year.
Earlier in July, a 36-year-old man in the Australian state of Queensland died after being attacked by a shark while spearfishing. And in June, a shark killed a 60-year-old surfer at Salt Beach near Kingscliff on the far north coast of New South Wales.