"As a result of the interview with the victim, other offending came to light. On one occasion, after a funeral on August 10, you hit him and threw him down the stairs," said Judge Harvey.
He said Dunn had told the court that he was angry with his son's behaviour and had admitted using an open hand and kicking him in the backside.
He said his son was causing him problems and he did not want to see him go down the same path that he did.
But Judge Harvey said violence was not the answer.
"We don't persuade a child from a life of crime by using crime as a means of persuasion."
He said child abuse was a problem in society.
"In Maori culture, children are described as taonga [treasure] but any culture would describe them as that because they are our future. They need to be nurtured, not bashed."
Judge Harvey gave Dunn a discount for his early guilty plea but said a term of imprisonment was the only answer.