Prime Minister John Key says it would be unacceptable for Families Commissioner Christine Rankin to take part in a campaign which aims to overturn the child discipline law.
A referendum will be held in July regarding changes to the Crimes Act in 2007 that removed the defence of reasonable force for anyone accused of assaulting a child.
A campaign was launched on Monday has been launched to urge people to vote "no" in a referendum about whether smacking should be a criminal offence.
The referendum question asks: "Should a smack as part of a good parental correction be a criminal offence in New Zealand?"
Ms Rankin has previously opposed the child discipline law, and media releases initially said she would attend the campaign launch.
Campaign organiser Bob McCoskrie later said that was a mistake but suggested Ms Rankin may have been muzzled on the issue. He says Ms Rankin should be able to speak out on the issue.
The Families Commission firmly supports the child discipline law and Mr Key says the commission usually speaks collectively.
"I don't think we should be absolutely pedantic about that; from time to time (Ms Rankin) might make the odd comment. But there's a big difference between that and broad characterisations of events and her actually actively campaigning would be unacceptable."
A group voting to keep existing child discipline laws says Ms Rankin should not be able to campaign to have the laws overturned.
The Yes Vote Coalition says Ms Rankin would be ignoring sound evidence that the commission bases its stance upon.
Ms Rankin has refused to comment on the issue.