A former All Black convicted of assaulting his son has had his conviction overturned on appeal to the High Court.

The man, who cannot be named to keep his son's identity secret, pleaded guilty in November after striking his son twice with a leather belt.

He was sentenced by Judge Tony Fitzgerald in the Auckland District Court in March to come up for sentence if called upon - in effect, no penalty - but his application for a discharge without conviction was declined.

The former All Black had argued he had completed an anger management course and the consequences of a conviction would be out of proportion to the offending.

He said he had engagements overseas and sat on a number of boards that could be jeopardised if he had to declare a conviction.

Also, it would be "embarrassing'' to declare his conviction at the border of foreign countries.

The District Court judge disagreed and entered a conviction, but the case was taken to the High Court on appeal.

In a judgment released late today, Justice Potter overturned the decision saying the man could be at risk of losing employment from speaking engagements, particularly overseas.

"Additionally, there is a real and appreciable risk that his continuing contribution to the community would be affected by the consequences of a conviction,'' she said.

In the appeal, the sportsman's lawyer John Eichelbaum sought to liken the case to that of a woman who was also discharged in the Court of Appeal for disciplining her son with a belt.

Justice Potter said the two cases had "singular circumstances'' and declined to use them in parallel.

She said the crime was serious but was at the lower end of the scale in terms of gravity.

The man strapped his son after his on-going fighting with his sister had caused "tension in the household''.

He struck the boy once on the legs and once on the shoulder with the tongue end of the belt, causing bruising and redness.

The marks were noticed at school and authorities informed.

Justice Potter said taking all factors into account, the consequences of a conviction would be out of proportion to the gravity of the offending.

She quashed the conviction for assault and discharged him without conviction.

Family First director Bob McCoskrie, who has opposed the "smacking law", said it was another case of the law targeting "the wrong type of parent''.

"If you talk to anybody, he's a good parent.''

McCoskrie said many parents convicted in the lower courts for disciplining their children were "rolling over and taking it'', but those who contested the decisions in higher courts were often successful.

He said the law was uncertain and parents were in a "grey area'' and legislation needed to be introduced to clarify exactly what was acceptable punishment.

McCoskrie said he supported MP Chester Borrows' bill that clarified the law - particularly that implements could not be used to discipline children - but that bill had been "shelved''.