Government claims that New Zealand will see an economic gain of $4.5 billion from the proposed 12-nation Trans Pacific Partnership (TPP) trade deal are "doubtful," according to an academic review released Wednesday.

The controversial TPP's economic benefits would be less than a quarter of those the government had claimed and the proposed trade deal, which is being negotiated in secret, would impose serious costs, said the report commissioned by the Sustainability Council of New Zealand.

The government had repeatedly used estimates of $4.5 billion in gains by 2025 from the US-based Peterson Institute to justify the TPP.

However, senior associate of the Victoria University Institute of Governance and Policy Studies Geoff Bertram concluded in a review that a third of the stated benefits should not be counted at all as they are outside established economic theory.

The review concluded that only a minor part of the remaining gains were justified and that the total benefits likely to be available were less than a quarter of gains Trade Minister Tim Groser had claimed, said a statement from the Sustainability Council.

"In exchange for a small gain in relative terms, New Zealand is being asked to sign away large slabs of its sovereignty. The TPP bundles the small gains from trade with a wide range of non-trade matters that will set privileges for foreign investors. These would impose serious costs in the form of limitations on a government's ability to protect the public interest," it said.

"More than any previous trade deal, it matters what gains are really available and whether these exceed costs. On the information publicly available, it is doubtful there is a net benefit for New Zealanders from the TPP."

The review stated that "a proper accounting will be possible only when a full text is made public."

The New Zealand government and negotiators have kept the texts of the proposed agreement under wraps, but Prime Minister John Key said last month that concluding the TPP was a"top priority."

The Employers and Manufacturers Association dismissed the review as "mere speculation, premature and wildly misleading."