The father, a Masterton resident, pleaded guilty to assault under a new law that allows parents to be convicted of using even "reasonable" physical punishment, The Dominion Post reported.

"A lot of us are parents, we know children can be challenging -- but we are the adults," Judge Anthony Walsh said.

The father hit the boy three times on the rear end in late October after he was told the 8-year-old had been acting up in school. He hit him hard enough to cause a bruise and the mother took a picture that eventually made its way to police.

Under the law, parents cannot argue reasonable force. But police guidelines say that "inconsequential" physical discipline shouldn't be charged, although experts say the courts will have to define what that means.