Dirty Politics author Nicky Hager is taking the New Zealand Police back to the High Court - this time over his travel information.

Mr Hager claims he has documents that reveal the police obtained his travel information from airlines to try to identify his confidential informants.

He alleges the warrant used by police to get this information was unlawful.

Otago University Dean of Law Mark Hanaghan told Rachel Smalley the law, rightly, does protect informants.

He said Hager has promised his informants confidentiality, and the law can't compel him to go to court and give them up - so he's saying the police shouldn't be able to get around that.

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