Winston Peters has been labelled a "dangerous old man" who's "really past his prime", after vowing to repeal the so-called anti-smacking law.
Sue Bradford, the former Green MP behind the law, told The AM Show on Monday she was "horrified" by his recent comments.
"What he's advocating is the return of the legalising of assault on our children, which is the last thing our kids need and the last thing the kids of Northland need."
The Northland MP and NZ First leader on Friday said: "We are going to repeal the anti-smacking law which doesn't work, and has in fact seen greater violence towards children."
Ms Bradford said: "He's talking about this on the back of the incident up in Kaikohe recently with the young people rampaging.
"Those kids probably see far too much violence I'd suggest in their lives already, far too much poverty, unemployment, a lack of opportunities for their families in their part of the country."
The 2007 law change removed the defence of "reasonable force" in cases where parents and caregivers were being prosecuted for assault on children.
"It's helped massively to change the idea that actually parents and other adults responsible for children are legally entitled to use physical punishment on their kids, that sometimes led to quite serious assaults," said Ms Bradford.
Repealing the law would send the wrong message, she believes.
"We've got 'it's not okay' campaigns about beating our partners, our wives, but on the other hand, children don't matter?"
Conservative lobby group Family First says there have been massive increases in child abuse in the decade since the law began, but Ms Bradford says repealing the anti-smacking law won't fix that.
"As the truly dreadful levels of family violence in this country continue, they cannot be laid to this law. No law can stop that."
The Government has no plans to repeal the law, nor will the National Party campaign to remove it ahead of this year's election.
"It's not on our agenda," Prime Minister Bill English told The AM Show.
"There was concern when the law came in, but there doesn't appear to have been any misuse of it or overreach, and we would see it as a backward step.
"Our view is that the smacking law remains in place."