A Christchurch father has been found guilty of assaulting his 4-year-old son after a two-day trial seen as a test of the anti-smacking laws.

After more than nine hours of deliberation, the Christchurch District Court jury last night found James Louis Mason not guilty of assault for lifting the bicycles that his two sons were sitting on and slamming them back down.

But on the third count - which accused him of pulling the 4-year-old's ear and punching him in the face - the jury returned a guilty verdict.

Mason denied all charges, but said he pulled the child's hair and flicked his ear to stop him going back into a dangerous situation on his bike on the Bridge of Remembrance ramp in central Christchurch where his 2-year-old had fallen and hurt his head.

Witnesses at the trial did not see the incident on the ramp. But they told of hearing Mason afterwards, swearing and shouting at the boys.

Judge Michael Crosbie remanded him on bail for a pre-sentence report and sentence on June 17.

He said he was considering whether a supervision sentence should be imposed to give him anger-management training.

Mason told One News outside the court he was baffled by the verdict.

"I'm not quite sure which [charge] I was convicted on ... I'd be interested to find that out. I'm just a bit dumbfounded at the moment."

Crown prosecutor Deirdre Elsmore handed up Mason's criminal record sheet, which showed no relevant convictions.

"I can understand that verdict and that it implies acceptance of the evidence of the people who were at the scene," Judge Crosbie told the jury.

He said he would normally have dealt with Mason, a 50-year-old musician, "on the hop" in a list court, but the wait for the jury's verdict had gone on until 9.30pm.

He asked for a pre-sentence report, but said he was not considering anything more sinister than the supervision and anger management.

Mason's lawyer, Elizabeth Bulger, said she might seek a discharge without conviction, and Judge Crosbie agreed not to enter the conviction on his record in the meantime.

"I am asking for a report out of an abundance of caution and out of respect for your children, more than anything else," the judge said.

Mrs Elsmore said it had been a short and straightforward trial involving legislation about which people tended to hold strong views.

She asked jury members to set aside their personal views, about smacking and parental control.

"This is not your opportunity to give Parliament any sort of message about whether this is a good law."

But Ms Bulger appealed to the jury members who were parents to remember what it was like to control toddlers.

TVNZ reported police in January saying Mason would have been charged with or without the legislation.

However, the legislation change prevented Mason's lawyers from using a reasonable force defence, Sue Bradford, the MP who drafted the legislation change said.

"Before June 2007, the jury may have said: "Look, it was fair enough he hit the kid in the face because he needed the discipline, whereas now, the jury can't say that," Ms Bradford said.

She said it was hard to evaluate the law change because it did not introduce a new offence, instead removing a potential defence.

"The thing that has changed is that police may be more willing to prosecute in cases like this than they were before 2007 because the chances of getting a conviction are higher," Ms Bradford said.

She said one case that stood out was that of a Timaru mother who beat her son with a horse crop and a cane on several occasions and was not convicted by jurors. She said that case was being heard as the law change was being argued in 2007.

Family First director Bob McCoskrie said last night that the conviction was appropriate if it was for punching a child.

But there was a concern that Mason may have been found guilty for only the ear-pull, as the actions of punching, and pulling the ear, were wrapped up in the same police charge.

"If that's the case, then it's a decision that does concern us. We would like that clarified to understand how the law is being interpreted by the police and the courts."

The anti-smacking legislation was passed by Parliament in May 2007, removing from the Crimes Acts the defence of reasonable force for parents who physically punish children.

Family First is campaigning for the repeal of the law and in March issued survey findings showing many parents were still confused about the law change.

As the law stands a light smack would not always be illegal. But 55 per cent of the 1000 people surveyed thought smacking was always illegal, 31 per cent thought it was not, and 14 per cent did not know.