Opponents of the Trans-Pacific Partnership (TPP) free trade agreement say it's going to lead to a flood of lawsuits against the Government.
The nine-country TPP is under negotiation and the United States is one of the countries that wants to join it.
NZ First leader Winston Peters says threats of legal action by tobacco companies if the Government goes ahead with its plain packaging policy are the tip of a litigious iceberg.
"The TPP empowers multinational companies to sue the New Zealand Government whenever the companies believe their interests are being threatened," he said on Friday.
"It locks in successive New Zealand governments who will forever be glancing over their shoulders as foreign barons circle with multi-million dollar lawsuits."
The FIRST trade union says the TPP has the potential to derail the Government's plan to force cigarettes and tobacco to be sold in plain packets.
"Under the TPP a tobacco company can complain it is hurting their profits and sue for compensation," said FIRST's general secretary Robert Reid.
The TPP is an extension of the free trade agreement between Brunei, Chile, New Zealand and Singapore which has existed since 2006.
Countries wanting to join it are Australia, Malaysia, Peru, the US and Vietnam. Japan is considering entering the negotiations.
There is no deadline for it to be ratified.