Activist and blogger Martyn Bradbury is pushing back against police trying to use secret evidence to defend searching his bank records without a warrant.

Police were found to have breached Bradbury's privacy by accessing the records in 2016 in their hunt for the hacker Rawshark.

They exploited a section of the Privacy Act which allowed agencies holding people's information to provide it without legal order if doing so would help "avoid prejudice to the maintenance of the law".

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Some banks treated these requests as red flags for customers, something Bradbury alleged happened to him, allowing police to "shred" his credit rating.

He earlier said the first he found out about his records being accessed was when he was denied a bank loan.

Bradbury ultimately suffered suicidal episodes due to the financial effects.

Now police are trying to defend accessing his records - but say the hearing should be held in closed court, using secret evidence.

That would mean Bradbury himself would not be allowed to see the evidence used by police in his case.

In a Human Rights Review Tribunal hearing in Wellington this morning, lawyer for the police Vicki McCall said the tribunal must have "inherent power" to receive secret evidence.

The tribunal wouldn't be able to determine whether means used to obtain the information were unfair or unreasonable without knowing what the circumstances of the case were, she said.

Police were "required" to withhold the information and without it they could not adequately defend themselves.

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If the tribunal did not allow a closed hearing, police's next move would be an application to strike out the case altogether.

"The claim should not, in fairness, be tried at all."

The step could mark the first time the tribunal accepted secret evidence in a closed hearing against the objections of the person who had brought the prosecution.

Bradbury's lawyer, Graeme Edgeler, said his client was in an "odd position".

"Had police actually done a lawful search and found the information that had linked him to Rawshark and prosecuted him, he would be entitled to this information. The reason he's not, is that he's innocent," he said.

"They're not going to be wholly incapable of defending this claim if they can't rely on this document."

He said Bradbury being told what type of information police were planning to use would go some way to "assuage his concerns".

"The blanket refusal to release this document - not explain what the document is, not explain the nature of the document - that is an interference with his privacy."

Journalist Nicky Hager received an apology after police obtained 10 months of his banking records.

Hager wrote the book Dirty Politics based on information allegedly hacked by Rawshark.

Barrister Felix Geiringer, who acted for Hager in the Dirty Politics fallout, said it was a matter of law that procedures in hearings which were hidden from other parties were "extraordinary" and only to be used in "extremely limited circumstances, if ever".

In Bradbury's case, the Privacy Commissioner John Edwards has already ruled police "were not justified" asking for the banking record, and the case should have been put before a judicial officer.

The Tribunal has reserved its decision.