Leader Andrew Little says the Labour Party will breach the Trans-Pacific Partnership if in government because it is the only way it can fight the deal.
Labour had its first briefing from Trade Minister Tim Groser on the 12-nation trade deal on Monday night.
Mr Little said yesterday the full impact of the deal was still not clear because the full text had not been released. He repeatedly refused to say if Labour would support or oppose it, even though it breached some of the party's bottom lines.
"The question for us now is what happens when we're in government."
Mr Little said Labour was unable to oppose the agreement in Parliament because negotiations were finished and no vote was held on its ratification. "The TPP is here now. There's no backing out of it."