The lawyer for Dirty Politics author and investigative journalist Nicky Hager says police conducted an invasive and chilling search of Hager's home during a raid last year.
Hager is seeking a judicial review into the steps taken by police during a raid at his home last October.
The raid was carried out following a complaint from blogger Cameron Slater, who claimed his emails had been illegally hacked.
The emails taken from Slater were then disclosed to Hager, and formed the basis of his tell-all book Dirty Politics.
The object of the police search was to discover the identity of the hacker who had disclosed information to Hager.
During the raid at Hager's home, a number of documents were confiscated, including physical records, computers, CDs, phones and USB drives.
Hager's lawyer Julian Miles QC told the court today that police actions were "invasive", and described the search as a "raid on the confidential information held by a journalist."
Mr Miles told the court the warrant police used, issued by a judge, sought a wider category of documents than just those linked with the hacker and those documents relevant to Dirty Politics.
"That's a further crucial issue which should have been discussed with the judge."
He said police had been given the right to "hoover" any of Hager's documents and this put the confidentiality of Hager's sources at risk.
"It's the search itself that is so chilling, coupled with the inevitability that the search will hoover up all information the journalist has in his possession."
Mr Miles has argued that police should not have applied for the warrant, as the source material for Dirty Politics was subject to journalistic privilege under the Evidence Act.
He said as an investigative journalist Hager has to promise confidentiality to his sources.
The need for journalists to protect the identity of their confidential informants was recognised by New Zealand statue and common law, Mr Miles said.
"At the heart of the success of any journalist ... is the ability to promise confidentiality and the ability to deliver that confidentiality ... the promise is not enough if you can't follow through."
He said if police were able to raid the homes of journalists, forcing them to provide the identity of their informants, it would have a "chilling effect" on the future relationship between a journalist and their source.
The hearing before Justice Denis Clifford is set down for three days.