Labour's acting leader David Parker says he believes Nicky Hager has a journalist's right to protect his sources and has questioned whether a 10-hour search of Mr Hager's home amounted to intimidation of the media.
Mr Parker said though he respected the independence of the police, it was crucial journalists were not forced to reveal their sources and police needed to tread carefully to ensure those rights were not breached. The Evidence Act gives journalists rights to protect information that may reveal their sources.
"While we respect the police's independence, we are concerned that an arm of the state appears to be being used against Mr Hager while nothing appears to be being done about the wrongdoing he exposed. A 10-hour search of their family home would be harrowing for anyone.
"Nicky Hager was doing what the fourth estate ought to do and police need to take care to protect his rights and to avoid the appearance of intimidating the media."
The police searched Mr Hager's home last week looking for information on the leaker dubbed RawShark who had provided him with hacked emails and social media messages between Whaleoil blogger Cameron Slater and his acquaintances.
Watch: Nicky Hager CHCH Book Launch
Those emails formed the basis of Mr Hager's Dirty Politics book, released before the election. Mr Slater laid a complaint with police over the hacking and the search was part of that investigation.
Prime Minister John Key said it was up to police to pursue the case as they considered appropriate and the Government could not get involved.
"It's appropriate they take the steps they deem necessary but wholly inappropriate for any politician to get involved."
Asked if he believed the law adequately protected people who were whistle-blowers, he said he didn't have advice on that.
"In the end people make their own calls and take own steps, but I haven't looked into that."
Mr Parker said he was pleased Police had confirmed they had taken proper steps to protect any claim Mr Hager had to journalistic privilege.
"I take some comfort from that."
However, he repeated his call that he would like to see police pay similar attention to investigating some of the issues the book had uncovered.
"Given this is politically charged, the police have to be very careful that they're not seen to be pursuing a political agenda whilst not paying due attention to the skulduggery alleged as a result of what Hager uncovered."
He said that included any potential illegality in the accessing of Labour's website or attempts to undermine then Serious Fraud Office head Adam Feely.
Mr Parker said Mr Hager's book had revealed serious alleged wrongdoings and resulted in a probe into whether former Justice Minister Judith Collins was involved in
undermining the Serious Fraud Office and whether the PM's Office was involved in misuse of power in its oversight of Official Information Act requests.
He said there were a mounting number of cases in which the police were called in to investigate media - including searches of media offices after Mr Key's conversation with then Act leader John Banks was recorded during the "teapot tapes'' saga and demanding the NBR hand over documents on South Canterbury Finance.
Journalist Nicky Hager's book Dirty Politics released, claiming underhand tactics by people within and close to the National Government to bring down their opponents.
Key figures were right-wing blogger Cameron Slater, aka Whale Oil, who was allegedly fed information from senior National Party ministers and staff, including Justice Minister Judith Collins and Prime Minister John Key's senior adviser Jason Ede.
A person using the identity Rawshark began contacting media and releasing emails and communications obtained from an alleged hack of Slater on Twitter with the handle @whaledump.
Cameron Slater laid a police complaint about the hack.
Justice Minister Judith Collins resigned after a leaked email claimed she had undermined Serious Fraud Office chief executive Adam Feeley.
Jason Ede quit his job at National's Wellington office the day before the Government was re-elected, according to party HQ.
Hager's home is searched by police believed to be seeking Rawshark.
Police said anyone with information related to the investigation could contact them, or phone Crimestoppers anonymously on 0800 555 111.