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The cable-tapping claim was raised by the Herald last month after the discovery of a visit by an NSA engineer to a GCSB base near Blenheim in February 2013. US Audit Office documents recording expenses paid by non-US taxpayers revealed the visit, describing it as a visit to set up a "Special Source Operations" site.
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The SSO is the division of the United States' National Security Agency which carries out cable tapping and collection of vast amounts of data from internet switching point. The data is used by NSA tools including X-Keyscore which is capable of sifting through the vast amounts of collected information to spy on individuals.
Greenwald told Radio NZ's Morning Report NSA documents showed the GCSB programme was underway and "phase one" involved accessing the underwater internet cable which connects New Zealand to the rest of the world. "Phase two" was the change in spy laws -- which happened last year -- which would mean it was legal for the programme to go ahead.
"Phase one entailed accessing that cable, tapping into it and then phase two would entail metadata probes - the sweeping up of large amounts of metadata indiscriminately in order to troll through it and analyse it and find out who New Zealanders are talking to," he said.
Greenwald says he will produce documents to prove cable-tapping went ahead at the Kim Dotcom-organised "Moment of Truth" at the Auckland Town Hall tonight.
At the time, a spokeswoman for the Prime Minister told the Herald that questions about the SSO visit appeared to be about "a cable access programme".
"We can categorically state that there is no such programme operating in New Zealand, and any claims that there is are utterly wrong."
In relation to plans for access of cables, she said "we are not and we have no intention of introducing one".
The conflicting claims are in direct conflict -- a specific point in an argument over mass surveillance. Key has said he canned a plan for mass surveillance in March 2013 because it was too intrusive.
The document which revealed the NSA visit was a General Accounting Office travel disclosure, in which NSA staff declare any travel costs not funded by the US taxpayer. In declaring a $271 payment for "transportation, lodging and meals", the NSA revealed it had sent an engineer to Blenheim for two nights to "participate in technical discussions regarding GCSB future SSO site".
The papers give no indication as to whether the tools are being offered to NZ's GCSB or whether they would be operated by NSA staff working in NZ.
The NSA already has an officer based in Wellington, according to a 2004 US Embassy cable in the Wikileaks files.
The engineer's visit was made to New Zealand as the country prepared to overhaul spy laws in a way which critics say gave them unfettered access to a massive amount of citizen's personal information.
While Prime Minister John Key said the new law tightened up the rules and better reflected the way they operated, critics claimed it gave the spy agency greater powers including the use of their powerful tools on Kiwis.
Greenwald told the Herald yesterday he had other GCSB-related revelations which would shed more light on New Zealand's role in the Five Eyes partnership. He said he hoped to get as much published before the election so people had more information before making voting choices -- but he also wanted to allow enough time for answers to be given to questions raised.
He said the "all-consuming alliance" of Australia, Canada, NZ, the UK and the US relied on technical know-how but also saw cash exchanges. "The GCSB spends a lot of money with the NSA in exchange for surveillance technology and equipment."
Greenwald dismissed claims by Mr Key that New Zealand was protected by the spy agency from Islamic terrorism. "On the list of things that are likely to kill you as a New Zealander, terrorism is so far down the list."
Cunliffe: Questions for Key
Labour Leader David Cunliffe said Mr Key had a series of questions about Greenwald's claims to answer including:
"Did the Prime Minister properly brief parliament when passing the GCSB law that there was work underway to develop mass surveillance of New Zealanders?
"Was it necessary to pass the TICS and GCSB bills to provide a legal pathway for the implementation of this project?
Mr Cunliffe said that if the Government was working on a large scale proposal to harvest New Zealanders' metadata, "Parliament should have been briefed on that when it was passing the law which it looks like the agencies saw as being a prerequisite for doing that project."
He also said Mr Key needed to be clear on whether the GCSB had "accessed metadata or data on New Zealand citizens through the Five Eyes networks from other Five Eyes partners which it would not have been lawful to obtain directly".
Asked about the Green Party's position, which is to shut down the GCSB's Waihopai spy base, Mr Cunliffe said that was not Labour's view, "and we are not saying is we would withdraw from Five Eyes".
"What we have said is that we think the Five Eyes arrangement must be within the scope of the thorough and far reaching review of our intelligence community that we will sponsor immediately upon taking Government."
Timeline
2011: Cyber attacks on New Zealand.
Early 2012: GCSB starts looking at a mass protection solution.
April 2012: Cabinet approves GCSB working towards a business case.
September 2012: Illegal spying on Kim Dotcom admitted.
October 2012: Then-Cabinet secretary Rebecca Kitteridge reviews GCSB.
February 2013: NSA engineer specialising in cable tapping comes to NZ.
March 2013: GCSB draft review tells Key bureau's practices don't fit with the law. Key puts GCSB "mass protection" plan on hold.
April 2013: GCSB review made public raising concerns about illegal spying on Kiwis.
May 2013: Two new spy laws are introduced. Key says it stops mass surveillance, opponents say it allows it.
June 2013: Edward Snowden begins to divulge NSA documents, which include the GCSB cable-tapping plan.
August 2013: After months of public meetings and concerns over mass surveillance, the new laws pass.