The Green Party says signing the Trans-Pacific Partnership could jeopardise protections for New Zealand's ability to pass laws in the public interest won in previous trade agreements.
The party says special clauses in those agreements, known as Most Favoured Nation clauses, could lead to the Government's freedom to regulate in areas such as tobacco being over-ridden by the TPP deal.
Greens' co-leader Russel Norman says New Zealand's trade deal with Hong Kong gives some protection to the Government's ability to introduce tobacco measures, but that is being put at risk by the TPP.
Dr Norman says New Zealand could be opening itself up to the sorts of lawsuits currently being faced by Australia from tobacco giant Philip Morris.
However, a former trade negotiator, Stephen Jacobi, says the Greens are assuming that New Zealand will back away from its position in those agreements in the current nine-country talks.
Meanwhile, the Green Party is accusing the Labour Party of siding with the Government on the Trans-Pacific Partnership deal.
Russel Norman says Labour has previously called for public scrutiny of the trade agreement, but questions asked in Parliament suggest that its stance has changed.
On Thursday in the House, Labour's trade spokesperson Clayton Cosgrove asked Trade Minister Tim Groser for assurance that any finalised trade deal would safeguard New Zealand's sovereignty.
Mr Cosgrove says he has been seeking a commitment from the Government that any agreement would protect New Zealand's rights to regulate and legislate for the public good.
Minister accused of misleading public
Russel Norman says Trade Minister Tim Groser is mis-leading the public over the Trans-Pacific Trade negotiations.
Leaked documents from the talks show New Zealand has agreed to give foreign companies the right to sue the Government over policies which cause them financial losses.
Mr Groser says the Government's right to legislate on social or environmental issues will be protected.
However, Dr Norman told Radio New Zealand's Morning Report programme on Friday that challenges to government decisions will be lodged with a tribunal, which will ultimately decide what it can and can't do.
Dr Norman says broader trade issues will over-ride domestic policies in the tribunal deciding such matters, as happened in the United States over the labelling of canned tuna.
He believes the public has been kept in the dark will not know what the Government is negotiating until four years after any deal is signed.
Warning NZ could lose protections over tobacco
An expert in international trade law says New Zealand could lose protections for tobacco regulation from previous trade agreements if it signs the Trans-Pacific Partnership.
Professor Jane Kelsey from Auckland University says clauses in previous trade deals allow them to be over-ridden by new trade agreements.
Professor Kelsey, a trenchant critic of the TPP, says clauses in some previous trade deals (Most Favoured Nation) clauses could allow those deals to be over-ridden by whatever is agreed in new trade agreements.
Anti-smoking group ASH says some of New Zealand's previous trade agreements have excluded tobacco control measures, including a trade deal with Thailand.
Professor Kelsey says protections agreed in previous trade agreements for the Government to legislate in the public interest could be in jeopardy, and lead to it being sued if the TPP takes a tougher line on tobacco.
Alcohol Action New Zealand has written to Trade Minister Tim Groser to express its fears about the Trans-Pacific Partnership agreement.
The group is worried the agreement could restrict governments from regulating the price and marketing activities of multinational liquor companies.
Spokesperson Doug Sellman says reducing alcohol abuse will come from law changes on price, advertising and accessibility, but the TPP agreement could stop such laws or expose New Zealand to litigation.