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Mr Key was responding to Mr Greenwald's comments on The Nation this morning that on Monday he would reveal information showing the GCSB had misled New Zealanders by claiming it had not conducted mass surveillance.
Greenwald, a Pulitzer Prize-winning journalist, said New Zealand's spying agencies had been conducting mass surveillance on New Zealanders as part of the Five Eyes arrangement between the US, the UK, Canada, Australia and New Zealand.
Mr Key said that was wrong. "There is no mass surveillance of New Zealanders by the GCSB and there never has been. Mr Dotcom's little henchman will be proven to be incorrect because he is incorrect."
He believed Mr Greenwald was jumping to conclusions based on partial information. Mr Greenwald has worked with Edward Snowden over material Mr Snowden obtained relating to the activities of spy agencies worldwide.
Mr Key refused to discuss Mr Greenwald's other claims about New Zealand spying on countries such as China or the programmes it used, saying he would not talk about New Zealand's spy work overseas.
"There are a variety of reasons why, in the international environment we collect information. But when it comes to New Zealanders, there is no mass surveillance, there never has been mass surveillance. Dotcom's little henchman is wrong and unfortunately, he might have hacked some information but not all of it."
Mr Greenwald will be speaking at Kim Dotcom's highly anticipated announcement, billed as the "Moment of Truth", on Monday.
He said he would respond to Mr Greenwald's revelations on Monday "in the fullness of time."
"But mark my words, I'm right and he's wrong and I'll prove I'm right."
Last year Mr Key said he would have to resign if the GCSB was shown to have conducted mass surveillance because it would have been illegal. That related to the new GCSB Bill which defined the circumstances in which the spy agency could spy on New Zealanders on behalf of the SIS or Police.
Mr Key said he still stood by that. He had been reassured by the GCSB they had not conducted mass surveillance.
He said the GCSB provided support to agencies such as the SIS and Police when required, but that was very targeted.
"It's primary aim is to gather foreign intelligence. It's done that over successive governments."
Mr Key said he did not know if the GCSB had received mass surveillance metadata on New Zealanders from its Five Eye partners, but he did not believe it had.
He also tried to attack Mr Greenwald's credibility for working with Mr Dotcom, saying Mr Dotcom was trying to "gerrymander the election."
"Let's understand what's going on here. Kim Dotcom is paying Glenn Greenwald to come to New Zealand a week before an election and he's trying to influence New Zealanders. Problem is, he's got his story wrong."
Under the law, the GCSB can not spy on New Zealanders unless it is on behalf of an agency such as the SIS or Police and it has a warrant to do so.
Mr Key said such cases included suspected terrorists and other issues relating to security.
New Zealanders should be entitled to privacy - Cunliffe
Labour leader David Cunliffe said the Prime Minister may not be fit for office if he has misled New Zealanders about the extent of mass surveillance they had been subjected to from its spying agencies.
This morning in Mangere, Mr Cunliffe told reporters that if evidence emerged that Mr Key had misled the public, it was "extremely serious".
"I would be extremely upset if the pledges that have been made to New Zealand around our freedom from mass surveillance prove to be false.
"If the Prime Minster of the country has lied to New Zealand, I expect New Zealanders to react in the ballot box.
"The Prime Minister is our only real check and balance on the intelligence establishment. This goes to his right to hold office."
Labour wants a full review of intelligence services and to repeal the GCSB law changes made last year, and replace it with a law that is "more protective of New Zealanders' rights to privacy and freedom".
Mr Cunliffe said the review would decide the appropriate level of surveillance, but the average New Zealander should be able to keep their metadata private.
"New Zealanders should be entitled to a right of privacy ... Those are all difficult issues and they'll need to be very carefully considered, but New Zealanders expect the Government to be straight with them and they expect their Prime Minister to tell the truth."
But he hinted that New Zealand could reduce the amount of information it shared with other countries at the expense of its relationships with America, Canada, the UK and Australia.
"New Zealand under a Labour-led Government always puts New Zealand's interests first and has an independent foreign policy, and that won't change under the next one."
Documents reveal communication interception - Greenwald
Changes to the law governing the Government Communications Security Bureau last year mean that the agency can now legally spy on New Zealanders, but Prime Minister John Key said that he would resign if the changes lead to mass surveillance of New Zealanders.
He made the comments last year in August when the law was being changed. He also said the GCSB had not collected "wholesale" communications metadata about New Zealanders.
Mr Greenwald said he has been working on documents for months, which revealed the interception of emails, telephone calls and all types of communications.
"The statement from the GCSB to New Zealand citizens last year that they do not engage in mass surveillance on New Zealanders is one that is not truthful.
"What I can tell you for certain is that the Government does engage in extraordinary amounts of analysis of metadata - meaning who's talking to who and for how long... on a massive indiscriminate scale.
"Not just internationally, but on New Zealanders as well."
He said New Zealand spent an "extraordinary amount of resources" for a country of its size on electronic surveillance.
"New Zealand spies on a variety of countries on behalf of the United States."
- with additional reporting by APNZ