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."
If it became the Government, Labour was unlikely to pull out of the TPP, which is New Zealand's first trade deal with the US and Japan. But Labour would "scrap out" with other TPP countries over some issues, in particular its policy to ban foreigners from buying property and land.
"If it means we end up breaching the TPP, we will scrap that out with the other TPP partners," Mr Little said. "It might be confrontational but it will be the only way we can fight it out."
Mr Little suggested Labour would support some of the TPP's enabling legislation, such as law changes to introduce lower tariffs, but he would need to see the details first.
Prime Minister John Key said it would be "bad faith" for Labour to legislate against the TPP's obligations. "But I can't bind a future government," he said.