A woman who admitted hitting her eight-year-old son has had her conviction for assault quashed by the Court of Appeal.

Lobby group Family First, a critic of anti-smacking law, says the case is an example of good parents being criminalised, contrary to assurances from politicians when the law was introduced in 2007.

The woman and her partner admitted they had smacked the child and were convicted in the District Court and sentenced to community work. The partner was subsequently cleared on appeal to the High Court.

The woman went to the Court of Appeal and the Justices said they had sympathy for the position she found herself in.

The woman had sought expert help to deal with the child's behavioural difficulties and had tried various non-physical measures.

The court said the penalties were out of all proportion to the gravity of the offending.

The woman has already served her sentence and lost her job.

Family First director Bob McCroskie said the case sent a warning to all parents about what they admit to authorities regarding smacking.

"This mother has had her career damaged, lost income and faced legal fees, and it's caused irreparable damage to the family.

"She was honest, asked for help, went to professionals who never came running with assistance but were quick to prosecute."