The New Zealand government conducted mass surveillance activities on its citizens despite denying such a project existed, NSA whistleblower Edward Snowden has claimed.

The infamous former contractor for the US spy agency revealed new documents which are said to prove that prime minister John Key mislead the public when he claimed that “there is no and there never has been any mass surveillance.”

“The [New Zealand spy agency] GCSB, whose operations he is responsible for, is directly involved in the untargeted, bulk interception and algorithmic analysis of private communications sent via internet, satellite, radio, and phone networks,” Snowden wrote in The Intercept on Monday.

The whistleblower said that during his time at the NSA he “routinely” came across the communications of New Zealanders via a mass surveillance tool, which GCSB contributes to, called “XKEYSCORE”.

Those top secret documents apparently prove that GCSB began a project with the NSA – dubbed “Speargun” – in 2012-13 which involved tapping the Southern Cross cable – the main undersea cable linking New Zealand with the rest of the world.

The second stage of the project – the illegal bit – was said to involve the insertion of “metadata probes” into the cables to monitor information about communications such as sender and recipient, dates and times.

However, doubts have been cast over the authenticity of the claims.

Anthony Briscoe, CEO of the Southern Cross Cable Network, which runs the eponymous cable, described the story as “total nonsense” and gave assurances that no metadata probes had been inserted.

“Let’s be quite blunt. To do this, we would have to take the cable out of service and I can assure you there’s no way we are going to do that,” he said in a prepared statement.

“It is a physical impossibility to do it without us knowing … There isn’t a technology in the world, as far as I am aware, that can splice into an undersea fibre optic cable without causing a serious outage and sending alarms back to our network operation centre, that something’s wrong.”

For his part, Key has admitted that a mass surveillance program was considered but ultimately rejected by his administration.