New Zealand spied on Japan to help the United States at a whaling forum, according to a document reportedly from the US National Security Agency.
The top-secret document from 2007 has been published as part of an article by website the Intercept, which says it was received from whistleblower Edward Snowden.
The brief report, seemingly from an NSA representative to the US Department of Commerce, says New Zealand collected and provided the US with "insightful" intelligence on the lobbying efforts of Japan in its bid to end a 21-year-old moratorium on commercial whaling at the 59th annual International Whaling Commission meeting.
"When the meeting ended on 1 June , the anti-whaling camp won, but the outcome was not clear going in," the paper says.
As US officials were "anxious" to receive the information during the actual negotiations in Anchorage, Alaska, an agent printed the information daily and drove it 30 minutes away to a locked conference room, where a small group of delegates read it in silence before negotiations.
"We knew the delegates valued the material simply because they took time from their very hectic schedules to be there and read it," the paper said.
The delegation included four representatives from the US, two from New Zealand and one from Australia, according to the document.
Comment has been requested from the GCSB.
Investigative journalist Nicky Hager has previously alleged New Zealand passed on intelligence information about other nations, including Japan, to the US.
Representatives from the Five Eyes intelligence agreement countries - including United States, Canada, Britain, Australia and New Zealand - are currently meeting in Arrowtown.