The Green Party and its Canadian counterpart have jointly expressed their concerns over the Trans-Pacific Partnership (TPP) trade deal negotiations - saying major corporations have had access to the draft text, while ordinary citizens are kept in the dark.
New Zealand is one of the four signatories of the deal, alongside Chile, Brunei and Singapore.
Seven other countries, including Canada, the United States and Australia, are negotiating to join.
Green Party co-leader Metiria Turei is in Canada, attending the Canadian Green Party's annual conference, and the two parties have discussed the trade deal.
Ms Turei says the New Zealand and Canadian Greens are "extremely concerned" that the deal will undermine governments, as it could open the door to them being sued by corporations unhappy at local laws, such as plain packaging of tobacco.
"More than just another trade agreement, the TPP provisions could hinder access to safe, affordable medicines, weaken local content rules for media, stifle high-tech innovation, and even restrict the ability of future governments to legislate for the good of public health and the environment," the two Green parties said in a joint statement.
"We believe that the process should be transparent. This agreement has been negotiated behind closed doors with a level of secrecy that is completely unacceptable in a democratic society."
The parties say representatives of major US companies and organisations, including telcos, pharmaceutical companies and the Motion Picture Association of America, have had access to the text, but non-government members of parliament and ordinary citizens are being left out.
They also warn that consumers outside of the US could end up paying more for books, DVDs and other items covered by intellectual property rights, because the TPP would let copyright holders veto parallel importing.
Large pharmaceutical companies could use the deal to deny overseas drug-buying agencies, like New Zealand's Pharmac, access to reliable, low-cost medicines, while harsher internet laws could also be required in signatories' countries, the parties say.
The Greens are calling for the government of New Zealand and Canada to follow Australia's lead and refuse to sign a deal that allows companies to sue over governments' policy decisions.