Senior Labour MP David Shearer has apologised for not toeing the party line on the Trans-Pacific Partnership (TPP) and says he won't do it again.
He is reported to have said he personally supports the 12-nation free trade agreement, although he will vote on party lines when enabling legislation comes to Parliament.
The Labour caucus has decided it can't support the TPP in its present form.
Leader Andrew Little says he instructed Mr Shearer to apologise to the caucus and he has done so.
"I have spoken with David and he has agreed his comments were careless and did not represent the decision made by the caucus," Mr Little said.
"He has assured me that he will now abide by the caucus position."
Another senior MP, Phil Goff, has broken ranks and is supporting the TPP.
Mr Little has allowed him to do that, saying he's a special case because when Mr Goff was trade minister he took part in the negotiations that led to the TPP.
"He's in a different position to David Shearer," Mr Little earlier told NZ Newswire.
"All caucus members know the importance of collective responsibility."
Mr Shearer is Labour's foreign affairs spokesman and is a member of the select committee that will examine the text of the TPP when it is submitted to Parliament.
Labour decided to oppose the TPP because it considers it to be an attack on democracy.
"It allows other countries and big business to have a say on how laws are made in New Zealand," Mr Little says.
The party will vote for some of the enabling legislation when the Government brings Bills to Parliament, but not for any measure it considers undermines democracy.
The TPP is due to be formally signed by trade ministers from 12 countries in Auckland on Thursday next week.
The TPP partners are New Zealand, Australia, Brunei, Canada, Chile, Japan, Malaysia, Mexico, Peru, Singapore, the United States and Vietnam.
Mr Little makes his state of the nation speech on Sunday.