The Police Association has hit back at suggestions investigations are influenced by politics, after a search of journalist Nicky Hager's house was ruled unlawful.

Photo: RNZ / Diego Opatowski

Opposition MPs have criticised police conduct after a High Court judge last week declared a warrant obtained to search journalist and author Hager's house was "fundamentally unlawful".

Justice Clifford found police failed to disclose relevant information to the judge who issued the search warrant.

Photo: Supplied

The police were investigating a complaint laid by blogger Cameron Slater, who said information from his computer, obtained by a hacker known as Rawshark, was published in Hager's book Dirty Politics. The book was critical of the National government.

But Police Association president Greg O'Connor told Morning Report it was "ridiculous" to suggest officers on the case had been influenced by politicians, and police would have behaved in the same way they did on all other cases.

"Anyone who thinks there is a politician sitting in the background pulling strings - the puppeteer - in any of these cases, it's wrong, it's ridiculous."

"Police would not have done anything differently," he said.

Mr O'Connor said police were extra careful in high-profile cases, such as Hager's.

"What actually happens is that as soon as one of these cases is high profile - and it won't be led by the politics, it will be led by the media interests often - is that everyone involved makes sure they don't get involved politically.

"If you're a detective, your job is to get all the evidence available because if you don't get it - because of the high media interest - you know that that case will be scrutinised to the nth degree, as all cases are, but of course the outcome will end up perhaps in a different place, perhaps in the paper."

He said police were never influenced by politicians.

"Any suggestion that there is any political person, whether it be an adviser or otherwise, is absolutely ridiculous."