The Government has blocked an attempt from Labour to have Parliament ask for the release of the full agreed text of the Trans-Pacific Partnership (TPP) free trade agreement before it is signed.
Labour leader David Cunliffe said he was "disappointed, but not surprised".
Prime Minister John Key said the Government had briefed Labour's trade and foreign affairs spokespeople on the TPP, but Cunliffe said a full and open public debate was needed.
There were definite downsides to the TPP.
Claimed benefits for New Zealand from the 12-country trade partnership, which includes the United States, have ranged between $5.5 billion a year by 2025 to $1b or less.
"We might pay more for medicines, we might be sued by tobacco companies for plain packaging," Cunliffe said.
"There may be benefits for New Zealand exporters in the agreement but without the release of the full text, we have no way of knowing their extent and nature.
"The Government must also reveal whether or not the text makes it more likely New Zealand could be sued by multi-national corporations such as tobacco companies."
Trade Minister Tim Groser said he would not negotiate the agreement "through the media".
"We'll look at any proposal, but the number one thing for the New Zealand government to ensure is that it doesn't limit any future New Zealand government's ability to legislate on public health."
Legislation over plain packaging for tobacco products is due to be debated today.
Groser said the Government was "fully expecting" a legal challenge over plain packaging regardless of the TPP agreement.
"Let's face it, Australia's in the middle of a legal challenge - TPP doesn't exist.
"So we'll never protect either an individual from an attack in a New Zealand court from a legal case, or a future New Zealand government from the possibility of a legal attack, but the key thing is to get the legal framework as robust as you can bat it back."
But Cunliffe said the public needed to be able to debate those issues as well as the trade upsides.
He said Labour had not made the full text public when it signed the free-trade deal with China in its last term in office.
But it had been more open than this government during the negotiation process by including non-government organisations and unions in full, open consultation as the deal was being developed.
He said Labour was reserving its position on signing the TPP until it saw the text and the fine print.
Labour was not asking for the text to be released before the TPP was finalised because that could interrupt the process, but it should be released before it was approved by Cabinet.
NZ First leader Winston Peters said Key was trying to keep the public in ignorance over the TPP.
"But New Zealanders deserve to know what rights are being negotiated away by this government," he said.
"The prime minister is misleading when he says the public will have the chance to have their say on the content of the TPP during the select committee process.
"They will have their say over the associated legislation that New Zealand will have to implement to agree to the terms of the TPP.
"But they will not get a say on the text of the treaty as that will have already been ratified by Cabinet."
- Fairfax Media