In a 2-1 decision dated Sept 8., the court ordered the new trial, ruling the trial judge applied a subjective standard concerning the duration of the spanking.
Court documents, which use only initials of family members, say the man, his wife, the six-year-old and the couple's two other children were driving to a museum in Fredericton on Aug. 1, 2009.
The boy was bickering with his brother, yelling at passing cars and kicking the back of his mother's seat as she was driving.
The father told the court that after efforts to reason with the boy failed, they stopped at a school parking lot and he spanked the boy, striking his clothed buttocks two or three times.
No bruising or marks were found when the boy was examined by social worker eight hours later.
Millicent Boldon, who lived in an apartment building near the school, testified that she heard a child crying and when she looked towards the school, she saw a man slapping a child.
She said the child was yelling, "You're beating me senseless. Stop. You're hurting me."
Boldon said she saw the man hit the child at least 10 times.
Boldon's brother -- Jim Burns -- said he also heard the child say, "You're beating me senseless."
He said that after an initial dozen blows, there were six more.
The Appeal Court decision said the trial judge applied a subjective standard when she said, "No spanking should go on and on to the point that strangers pick up the phone and call the police."
It also said the judge failed to properly explain the reasons for her ruling.
"She did not state why the accused's evidence did not raise a reasonable doubt, or why the remaining evidence convinced her of the accused's guilt beyond a reasonable doubt."
The Criminal Code states that force may be used to correct a child as long as it does not exceed what is reasonable under the circumstances.
Justices Richard Bell and Wallace Turnbull allowed the appeal while Justice Alexandre Deschenes filed a dissenting opinion.
"In my view, the reasoning adopted by the trial judge to reject the defence raised by the appellant is unequivocal and her decision is based on facts amply supported by the evidence she accepted," Deschenes wrote.
A date for a new trial has not been set.