Trade Minister Tim Groser says New Zealand won't break ranks over the Trans Pacific Partnership, despite calls to release details of the agreement now.
Politicians from seven of the 12 countries involved in the Trans Pacific Partnership are demanding the text of the proposed agreement be released.
In New Zealand, the Greens, New Zealand First, and the Maori and Mana parties have all endorsed their statement.
Mr Groser said the text will not be made public until it goes before Parliament.
He said that if details are released now it would be harder for countries to compromise as it would be seen as losing face.
Concerns have also been raised about the TPP jeopardising legislation to introduce plain packaging for cigarettes.
But Mr Groser said the deal is just one of many platforms which ruthless tobacco companies will try to use to stall the law.
The Government says the public will get a chance to have its say once a deal is concluded.
Govt assurances won't satisfy NZers - Cunliffe
But Labour Leader David Cunliffe said assurances from the Government that the deal would not limit sovereign rights were not enough to satisfy the New Zealand people.
Big corporates, including tobacco companies, have been lobbying furiously to be able to sue governments which might limit their profits for reasons such as public health, Mr Cunliffe said.
"It's absolutely essential that the rights of both current and future governments, to regulate and legislate in the public interest, must be sovereign and must be protected.
"So we're going to be looking very, very carefully at that text, and that is exactly why we're challenging the Government to put it in the public domain as soon as it is negotiated and before it is signed," he said.
Greens co-leader Russell Norman said he hoped the Government would take notice of the open letter, as people needed to know what was in the deal so they could have a say before it was signed.
"The Government says (it) can't release the text because it will interfere with the negotiations and it will tell the other parties our negotiating position," Dr Norman said.
"The other parties in this negotiation already know what's in the text. The only people who don't know what's in the text are the citizens of the countries involved."
Hard to estimate how much TPP will benefit NZ - Groser
Tim Groser said the proposed deal will benefit New Zealand, though he admitted it was to hard to say by how much.
The claimed benefits from the TPP range from more than $5 billion, to very little, if anything at all.
Mr Groser has told an APEC business leaders meeting in Auckland it was hard to estimate the gains of the proposed deal, and it could be less or substantially more than $5 billion.
He admitted most studies overestimate the benefits.
But Mr Groser said New Zealand's trade agreement with China has exceeded all expectations and fostered greater investment between the two countries.