A Christchurch musician has successfully had his conviction for assaulting his four-year-old son quashed by the highest court in the land.

The Supreme Court said today there would be no retrial ordered.

James Louis Mason, 53, was found guilty at a Christchurch District Court trial last year of punching his son in the face and pulling his ear in December 2007.

He was acquitted on two other assault charges.

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 two-year-old brother had fallen and hurt his head.

He was sentenced to nine months supervision and an order to undertake programmes as directed by the Probation Service.

He then unsuccessfully appealed his conviction at the Court of Appeal before taking his case to the Supreme Court.

In its decision released today, the Supreme Court said Mason's conviction must be quashed as two counts of assault had been included in one charge.

"The special nature of the present case is the distinctly different nature and seriousness of the two alleged assaults and the practical possibility of a particular defence for one of them only," it said.

The inclusion denied Mason the chance to clearly defend the act of ear-pulling.

"It was a jury question whether that action (if proved) was a use of reasonable force to prevent or minimise harm to the child by stopping him from riding his bike down the ramp...

"But it was exceedingly unlikely that the jury would be of that view if it concluded that the child's face had been punched," it said.

The case was earlier considered a test of anti-smacking laws.