A District Court judge and police say the prosecution of a man who insisted he had flicked his son's ear, only later to be convicted of punching the child in the face, was never a test case for child smacking laws.

Christchurch musician Jimmy Mason was yesterday sentenced to nine months' supervision and placed on an anger management course for the 2007 assault.

The incident happened when Mason's 2-year-old son injured his head biking down a ramp on Christchurch's Bridge of Remembrance. Mason said the boy "got clotheslined by a rail at the bottom of the ramp" and ended up "bleeding and moaning".

He said he pulled at his 4-year-old son to come with him to get ice from a dairy for the 2-year-old's injury.

"I flicked him on the ear and pulled his hair to stop him buggering off," Mason said. But he denied punching the boy in the face.

Mason was found guilty at trial of pulling the 4-year-old's ear and punching him in the face. He was acquitted of two other charges of assault in respect of the 4-year-old and another son.

The case was widely seen as a test of the anti-smacking laws because Mason publicly claimed that he had done no more than administer a flick on the ear.

But Christchurch District Court Judge Michael Crosbie said the matter would have been dealt with in exactly the same way before the introduction of the smacking legislation.

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

"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."

Judge Crosbie 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."

Christchurch police central area commander Inspector Derek Erasmus was just as blunt.

"This was not a test case. It was simply a case of a child being punched in the face and the police taking appropriate action."

Mr Erasmus said the sentence vindicated 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]."

Mason's wife, Ann, said there could be grounds for an appeal because the charge of assault combined the two alleged offences of an ear-flick, which Mason admitted, and punching his son's face, which he denied.

Mason's lawyer, Elizabeth Bulger, declined to comment on a possible appeal.