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A Christchurch father was today sentenced to nine months' supervision for assaulting his four-year-son in 2007.

Musician James "Jimmy" Mason, 50, was found guilty last month of punching his son in the face and yanking his ear at Christchurch on December 19, 2007.

Judge Michael Crosbie said Mason's actions were not discipline but an angry parent's reaction to a stressful situation.

He ordered Mason to attend anger management classes.

Judge Crosbie said the case was described by the crown as an inappropriate response in a stressful parenting situation.

The children had gone down a ramp on a bridge on their bikes and one had fallen and hurt his head. They were then scolded and one of them was assaulted by Mason, the jury found.

"It remains a case of an assault on a child, as it would have prior to the legislation being enacted," the judge told Mason.

"You don't accept the facts but you do say that in hindsight you should have been able to manage the situation in a calmer manner and not reacted in the way you described."

He said a discharge without conviction was not appropriate. "It is clear the jury found there was a punch and you admitted as much to the police."

Inspector Derek Erasmus of the Christchurch Police said police were very pleased with the result.

Mr Erasmus said it validated the police decision to prosecute Mason.

"For police this was never an issue about a flick to the ear or a pull to the ear as portrayed by some parties with an interest in this.

"This was simply a case about an adult punching a child and that is what he was convicted of.

"The law in relation to child discipline had nothing to do with the police decision [to prosecute]."

Mr Erasmus said, as the judge had pointed out, Mason would likely have received the same result under the old child discipline law.

"Some people have attempted to make this case into a wider issue but it was very clear from the judge in court this morning that it was very simple and turned on its facts."

Mr Erasmus refused to comment on how police felt about the sentence.

Outside the court today, Mason continued to deny pulling his son's ear.

He described a policewoman's evidence as "a pile of crap".

- With NZPA