The spy watchdog is set to investigate why blogger Cameron Slater was given declassified Security Intelligence Service documents refused to reporters.
Investigative journalist Nicky Hager's book Dirty Politics contains allegations that Whaleoil's Slater was tipped off by Prime Minister John Key's staff to ask for the papers.
SIS gave the briefing notes to Slater within a week of his OIA request, but declined to release them to the Dominion Post.
The Inspector-General of Intelligence and Security will now investigate after concerns were raised by the Green party.
"John Key can no longer say these dirty politics allegations are 'baseless' and a 'conspiracy'. The IGIS thinks it legitimate enough to investigate," Green Party co-leader Metiria Turei said.
"John Key's strategy of attacking the claims and denying wrongdoing has failed because now an independent authority has found them worthy of investigation."
The documents embarrassed then-Labour leader Phil Goff who had wrongly denied he was briefed by SIS boss Warren Tucker. Slater wrote a post in August 2011, accusing Goff of lying.
Hager's claims were based on emails and messages hacked from Slater's computer, and Slater has laid a complaint with police.
"The allegation that the NZSIS declassified a document for the express purpose of releasing it to a prearranged source as part of an orchestrated political attack is very serious and it is right that the IGIS will investigate," Turei added.
Key has repeatedly denied his office asked for the OIA request to be fast-tracked.
Today he welcomed the investigation.
"I think that is very good news," Key said.
"At the end of the day, we are very confident in the position [that] the SIS themselves have been running that process themselves independently, and they have been commenting that everything was absolutely above board.
"But the really good thing is the Inspector General will be able to have a look and clarify it for everyone."
The Privacy Commissioner has declined to investigate the allegations around Justice Minister Judith Collins' release of information about civil servant Simon Pleasants or reopen the investigation into the Bronwyn Pullar leak, the Greens said.
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