The most instrusive and powerful tools in the United States' electronic spying armoury were being lined up for installation in New Zealand last year, according to a document obtained by the Herald.
An engineer visited a GCSB base near Blenheim in February 2013 to talk about setting up a "Special Source Operations" site.
The SSO is the division of the United States' National Security Agency which carries out cable tapping and has vast resources to trawl and capture massive amounts of internet content and electronic communication, including the PRISM system.
Disclosures of its powers were the most controversial of those made by NSA whistleblower Edward Snowden when he began revealing its capabilities last year. Use of the powerful tools has created diplomatic rifts and forced US president Barack Obama to talk of reigning in his electronic spies.
The GCSB has refused to say whether it went ahead with plans to install the SSO site.
But, asked directly about the engineer's visit, said: "From time to time we discuss contemporary technology with partners, including where it may or may not be useful."
The GCSB spokesman also pointed to a recent review of the bureau, which said New Zealand's intelligence partnerships gave it "much greater and more valuable support, skill, technology and intelligence than would otherwise be available to it".
The spokesman also repeated the bureau's position saying "we don't do mass surveillance and we don't use partners to circumvent the law".
A spokeswoman for the Prime Minister said there no cable tapping operation was operating in New Zealand and there were no plans to introduce one.
She said NZ did not use the term "Special Source Operations" and made no comment about the other capabilities, which include accessing material through the Prism system or corporate partnerships with major telecommunications companies.
She said the GCSB Act was changed to give the agency a clear legal framework and stronger oversight.
The document which reveals the spying 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.
Green Party leader Russel Norman said the technology used at an "SSO site" was the equivalent to establishing "a total surveillance state".
He said the Prime Minister needed to tell Kiwis if the plan for a "future site" and if the powerful spying apparatus was now operating on New Zealand soil. He said the Prime Minister should also make clear it was an NSA site operating on NZ soil or whether it was the GCSB using US technology.
Mr Norman said Mr Key must have known as minister of the intelligence agencies.
"It's extraordinary Key knew this was happening all through the whole debate around the GCSB legislation without making reference to it. At the same time, he was approving total surveillance of all internet and phone traffic in and out of New Zealand."
The document containing the SSO reference was identified and forwarded to the Herald by Denis Tegg, a lawyer with an interest in civil rights issues.
The line in the documents which caught Mr Tegg's eye was from information originally sourced by US reporter Jason Leopold.
Where has the #NSA been traveling to since the Snowden leaks? Here's their travel reports https://t.co/9QnJzrfmIG https://t.co/rFeaAnAb32
Mr Tegg said he was concerned at the reference to an SSO site in New Zealand. "It means there is the potential for everything New Zealanders do on the internet to be accessed on New Zealand soil.
"If there is a site in New Zealand - or being contemplated - we have had no discussion about that happening. It seems pretty clear the amendments to the GCSB Act was designed with this in mind."
Mr Tegg said the reliance of some of the systems on compliance by telecommunications companies required Telecom and others to come forward about approaches they had from the government. He said the government also had to be open about the tools it was using.
"We have control over this - it's on our soil. We're entitled to know what's being done in our name."