Trade Minister Tim Groser acted unlawfully in withholding some information about the Trans-Pacific Partnership, the High Court has ruled.

Anti-TPP activist and University of Auckland academic Professor Jane Kelsey and others sought a judicial review of Mr Groser's decision to reject an Official Information Act (OIA) request for eight categories of documents which related to the 12-nation trade deal.

In a judgement released this morning, Justice David Collins said Mr Groser failed to follow the rules set out in the Official Information Act before refusing her request.

"When the Minister refused Professor Kelsey's request, neither he nor his officials assessed each piece of information requested against the criteria in the Act for withholding official information.

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"Instead, the Minister adopted a 'blanket approach' to the request based upon his knowledge of the categories of documents requested by Professor Kelsey. I have concluded this approach did not comply with the Act."

Mr Groser was told to reconsider his response to six of the eight categories of information.

"When the Minister reconsiders his decision he will be required to do so in a way that is consistent with his obligations under the Act."

The ruling could also have implications for the Ombudsman's office, which upheld Mr Groser's decision not to release the documents.

Justice Collins said he made the order for two reasons.

Firstly, there was "no lawful basis" for the Minister to withhold some of the information requested by Professor Kelsey.

Secondly, Justice Collins wrote, "the Act plays a significant role in New Zealand's constitutional and democratic arrangements".

"It is essential the Act's meaning and purpose is fully honoured by those required to consider the release of official information."

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Professor Kelsey said her decision to seek a judicial review had been vindicated.

"The Minister's approach epitomises the contempt for democratic processes and accountability that has pervaded these negotiations," she said in a statement.

But a six month deadline imposed by the court for further orders was "cold comfort" because it would fall after the negotiations had been completed.

"[Mr Groser's] unlawful approach in circumventing the Official Information Act appears to have achieved its goal," Professor Kelsey said.