The Labour Party says the Government has been too secretive about talks to set up a Pacific-wide free trade area.
The Trans-Pacific Partnership (TPP) is on the agenda at Labour's annual conference which begins on Friday.
Supporters of the partnership fear the party will end the bipartisan political approach to free trade by opposing any deal.
Labour's trade spokesperson Phil Goff is a strong advocate of free trade but says the party is concerned about the lack of transparency surrounding the TPP talks.
Mr Goff says National has not been inclusive or open about the position it is taking on key issues like Pharmac, the right of Parliament to regulate and intellectual property.
"We're not expecting the Government to disclose negotiating texts while they're being negotiated but we are expecting them to be upfront and open about the issues and about the stance that they are taking on our behalf."
Trade Minister Tim Groser says a shift by the opposition could undermine the talks.
Lobby group the International Business Forum says New Zealand firms and workers cannot afford to be left behind if a deal is struck.
The 12 countries in the TPP have a combined annual economic output of more than $33 trillion.