Mr Key said the Security Council bid was started by former Labour Prime Minister Helen Clark who told him when she handed the job over to him that he should pursue it with vigour.
"We are a good country doing good things. This guy turns up ... he's not a passionate New Zealander.
"He's a conspiracy theorist and people will see it for what it is, " Mr Key told reporters last night.
He said he did not know if Mr Greenwald's promised revelations would cost New Zealand Security Council votes.
"It would be a tragedy if it didn't happen because of that."
He said most countries had foreign intelligence agencies of their own and he did not think any of the countries Mr Greenwald could be preparing to name would be surprised or shocked that New Zealand had spied on them.
The GCSB could get involved for a range of reasons, from a New Zealand sport team preparing to travel to a risky country where intelligence was needed or for a specific reason such as freedom fighters going overseas.
Mr Key also invoked Ms Clark's name to bolster his assurance that there had been no mass surveillance.
"The simple question for New Zealanders is who do you want to believe? Do you want to believe the independent Inspector General, the current director of the GCSB and the former one, Helen Clark and me - five passionate New Zealanders.
"Or do you want to believe a couple of guys who have been brought out here by Kim Dotcom and don't have New Zealand's best interests at heart."
He was bound by the responsibility of being Prime Minister and could not reveal everything the agencies were doing.
"There is an element of 'trust me.' I'm not doing anything different from Helen Clark other than up cyber security because the risk has become much greater.
"In the end, if it costs me a few votes I'll still rest easy on that because I know I've done the right thing for New Zealanders."
Labour leader David Cunliffe has promised to review the intelligence agencies and believed Mr Key should have revealed the proposed cyber security measure earlier.
Mr Key also stuck to his belief that the NSA was not spying on New Zealand, saying it had higher priorities to worry about. "If Barack Obama wanted to know something about New Zealand I suspect he'd just give me a ring."
Mr Greenwald is the journalist whom former NSA analyst Edward Snowden entrusted with his stolen documents detailing mass surveillance by the US foreign intelligence agency.
Mr Greenwald and Mr Snowden appeared at an event in Auckland on Monday sponsored by Mr Dotcom, the Internet Party founder who is fighting extradition to the US on Internet piracy charges, to allege the GCSB conducted unlawful mass surveillance on New Zealanders, which Mr Key vehemently denies.
