Jack Westbrook was at his home, about 35km south of Nelson, when the woman, a friend of his older brother, came to visit with an American bulldog last week.
A neighbour saw the woman being mauled by the dog as she was putting it into a kennel on the property. The neighbour cried for help, alerting Jack and his mother Nicole to the attack.
A king hit to the dog's head using a punch learned from a year of Shaolin Kempo training sent the animal staggering back, he said.
"It took a few uncoordinated steps, then just looked at me."
Mrs Westbrook said the victim did not want to comment on the attack or reveal what injuries she suffered but was grateful to Jack for intervening.
Tasman dog control officer John Bergman said animal control experts arrived after the attack was over and they did not know exactly what happened because the victim refused to talk about it.
As far as he was aware, the dog wasn't registered but was owned by the woman's husband. It was shot at the scene. This is the negative impact that our dog laws have created. She didn't want to complain BECAUSE it was a dog she knew. If she complained, the dog's owner (ie her husband) would go to jail.
The dog was shot at the scene, and that is the end of that. When dog 'control' officers and the law come in, that's where problems start.
I hope for the women's sake, this is the end.
Halifax Veterinary Centre veterinarian Hans Andersen said Jack's actions were "courageous".
There had been incidents of American bulldogs killing people in the United States, he said.
Mrs Westbrook said she was incredibly proud of her son.
"He's my hero."
What a great story... great that someone came to the rescue. A lot of people would have shy-ed away.