The Acting Inspector-General of Intelligence and Security has determined that New Zealand spies helped the NZ Defence Force (NZDF) try to identify a source in a journalist's book.
Investigative journalist Nicky Hager claimed the New Zealand Security Intelligence Service (NZSIS) assisted the NZDF in efforts to identify sources for his 2011 book Other People's Wars.
NZSIS director-general Rebecca Kitteridge has apologised to Hager for "these failings, any impact they had on him, and any distress that has been caused".
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The Acting Inspector-General, Madeleine Laracy, said in her report the NZDF came to the conclusion that Hager's book contained classified information which may have been disclosed by a Defence Force officer.
She said the NZDF came to suspect a particular officer, but its inquiries were inconclusive, so it sought assistance from NZSIS personnel to take the investigation further.
The NZSIS then conducted a "preliminary investigation" for NZDF. But after analysing Hager's book, the spies did not come to any conclusive result.
Laracy said the NZSIS "acquired three months of telephone call metadata for the NZDF officer's home and mobile numbers and two months of call metadata on Hager's home telephone line".
The NZSIS advised the Defence Force that its inquiries had been inconclusive as to any connection between Hager and the Defence Force officer, and the NZDF decided against any further investigation.
But Laracy said she found the NZSIS assistance to be "unlawful".
She said the NZSIS provided that assistance despite a lack of grounds for reasonable suspicion that any activity had occurred that was a matter of "national security".
"I have been unable to find that the [NZSIS] showed the kind of caution I consider proper, for an intelligence agency in a free and democratic society, about launching any investigation into a journalist's sources.
"To the extent that Hager was the subject of NZSIS inquiries that I have found were not within the lawful scope of NZSIS activity at the relevant time, I consider he was adversely affected by the agency's activities."
Hager's book concerned New Zealand's military and intelligence activity in Iraq and Afghanistan.
Laracy's report follows the findings of an investigation she released last month into whether New Zealand intelligence agencies were involved in a CIA torture programme.
She found no evidence New Zealand was involved in the Central Intelligence Agency's (CIA) detention and interrogation programme.
But she found the NZSIS and Government Communications Security Bureau (GCSB) did receive information from CIA detainee interrogations, which often involved torture methods.
Hager is known for books such as Dirty Politics, published in 2014, which was based on emails hacked from Cameron Slater of right-wing blog Whale Oil.
It contributed to the resignation of Judith Collins as a minister.