In a post published on The Intercept website on Monday afternoon, Edward Snowden — a former analyst at the US National Security Agency (NSA) — says he came across the communications of New Zealanders during his work.
"If you live in New Zealand, you are being watched," he wrote.
Prime Minister John Key's claim that there has never been any mass surveillance by the Government Communications Security Bureau was false, Snowden said.
"At the NSA, I routinely came across the communications of New Zealanders in my work with a mass surveillance tool we share with GCSB, called XKEYSCORE.
"It allows total, granular access to the database of communications collected in the course of mass surveillance. It is not limited to or even used largely for the purposes of cybersecurity, as has been claimed, but is instead used primarily for reading individuals' private email, text messages, and internet traffic," Snowden said.
"I know this because it was my full-time job in Hawaii, where I worked every day in an NSA facility with a top-secret clearance."
In response, Key has released a tranche of New Zealand Cabinet documents that the PM says is setting the record straight, The New Zealand Herald reported.
"Claims have been made tonight that are simply wrong, and that is because they are based on incomplete information," Key said. "The GCSB does not collect mass metadata on New Zealanders, therefore it is clearly not contributing such data to anything or anyone."
John Key has previously stated that he will resign if mass surveillance has taken place in New Zealand.
American journalist Glenn Greenwald is expected to release more details about the GCSB at Internet Party founder Kim Dotcom's "moment of truth" event in Auckland on Monday night.