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In a 10-hour search of his house, Hager said computers and papers were seized in what appeared to be an attempt to discover the identity of the person who provided information used in the Dirty Politics book.
The book was an election bombshell based on hacked email and social media material belonging to WhaleOil blogger Cameron Slater.
The person contacted the Herald and Fairfax using the identity Rawshark while using the Twitter handle @whaledump to release information publicly.
Hager said five officers came to his Wellington home last Thursday with a search warrant.
He was in Auckland at the time the police arrived giving lectures at the University of Auckland.
"Soon after the police arrived, the lead detective stated that I was not a suspect in their case, merely a witness." Hager said he told the detective there was nothing in his house which held information that would uncover the source.
"Nonetheless, he and his four colleagues seized a large collection of papers and electronic equipment belonging to my family, including computers, drives, phones, CDs, an IPOD and a camera."
Hager said the search and seizure of the material was a "fishing expedition" carried out by officers who had no idea who they were looking for, hoping for a lucky break.
"I am confident that the police took nothing that will help them with their investigation."
Hager said he would not cooperate with police in any way to reveal the Dirty Politics source - or any other source. "I believe the police actions are dangerous for journalism in New Zealand.
"It matters to all people working in the media who could similarly have their property searched and seized to look for sources. People are less likely to help the media if the police act in this way.
"The police want people to respect their role in society; they should in turn respect other people's roles in society."
He said he was speaking to his lawyers about challenging the police action.