A father convicted of assault after smacking his son on the bottom has been told by a judge that parents cannot get away with such behaviour any longer.

The eight-year-old boy suffered a bruised shoulder after his 33-year-old father pulled him onto a bed and bent him over his knee, smacking him three times with an open hand across the buttocks.

The man, whose name was permanently suppressed, was sentenced to nine months supervision including anger management at Masterton District Court yesterday.

Today interest groups were split on whether the court decision was proof of protection for children or evidence of parents being unfairly targeted.

Police claimed the boy also suffered bruised buttocks, but the defendant disputed this, the Wairarapa Times-Age reported.

The man had arrived at his home in Masterton from work in the evening, to be told the boy had been in trouble at school.

He had confronted the boy in the bedroom where the offence occurred.

He later told police he had over-reacted and lost his temper.

Judge Anthony Walsh told him, "one time, maybe, you could have got away with this, but you can't do that now".

Judge Walsh said there were "other ways of disciplining, short of violence."

"You must understand that what you did amounted to an assault. Our law has been amended so that children are protected."

Judge Walsh asked the man's partner, who was seated in court with a victim's adviser, how the boy was now.

"He's playing on it, like, 'you can't sit next to me, Dad,'" she said.

The woman said her partner was still relating to his son, "but not the disciplining - I'm doing that."

"As a parent, all parents know children can be challenging," Judge Walsh told the defendant.

"But you're the adult, and you need to take responsibility and sort it out."

"As long as he knows there are certain rules he must comply with and if not there are consequences, for example time out and loss of privileges, then that's the way to go."

Child advocacy group Barnardos New Zealand today welcomed the court's decision.

Acting chief executive Peter Gerrie said: "I believe this is the first reported case in which the new law relating to child discipline in the home has been tested in court and the law has worked well.