Greenwald, invited to New Zealand by Kim Dotcom, told Q+A New Zealand spies "not just on [enemies] but on western democracies as well".
A "huge pile of evidence" shows Five Eyes spies for economic advantage against companies such as Brazilian energy company PetroBras.
On Saturday, Mr Key said he suspected Greenwald's mass surveillance claims were "part of a conversation" of a surveillance plan that was never formulated.
Mr Key said "there's total mention" the GCSB was considering something akin to mass surveillance, but "without a shadow of a doubt GCSB does not conduct mass surveillance on New Zealand".
Watch: Key: GCSB passes on Drone intelligence
Mr Key likened the GCSB to "a Norton anti-virus at a high level", saying it works against malware but needs a warrant to look at email.
"Last weekend, Spark was brought to its knees because some people clicked on malware which brought the network down. Cyber risk is exponentially rising."
"This is about protecting secrets rather than getting secrets."
Mr Key said he is doing nothing differently to Helen Clark and said the GCSB provides protection from fighters in Syria and Iraq, saying "Hell, yeah" those countries presented a risk to New Zealand.
He also claimed the GCSB helps to protect New Zealanders overseas, such as the All Blacks and other sport stars.
Mr Key repeatedly described Greenwald as a "henchman" of Dotcom, and said he didn't have the full information.
"This is the problem when you wander into New Zealand six days before an election and do Dotcom's bidding and get half the story."
Mr Cunliffe said he had been assured that blanket surveillance was not going on and if it was, "the Prime Minister would have utterly serious questions to answer".
He agreed some security threats need to be watched, but Mr Key's mention of Spark was a "mix up".
"I am not saying there is no place for a properly-conducted cyber authority - that is why we have a policy on a Digital Bill of Rights," Mr Cunliffe said.
He also argued that "New Zealand gains more than it contributes" from the Five Eyes arrangement.
Both Mr Cunliffe and the Greens co-leaders said they would conduct a review of the GCSB if elected to Government, with Dr Norman promising to "close Waihopai [spy base]".
Pulitzer Prize-winning journalist Greenwald, meanwhile, said it would be New Zealand citizens who decide if documents show mass surveillance is going on.
"Any politician who has stated there is no mass surveillance is a politician who has been deceiving the public," he said.
"That's what my documents will show."
Greenwald said the type of metadata collected equating to mass surveillance could monitor internet searches around infidelity or abortions, for example.
Mr Key was "wrong" to use the public's fear over cyber protection to justify spying, the Green Party has said.
Following the Prime Minister's comments on Q+A this morning, Dr Norman issued a statement saying Mr Key was "trying to scare and confuse New Zealanders by saying mass surveillance by the GCSB is needed to achieve the country's cyber protection".
"Our cyber protection is undermined by being carried out by spies," Dr Norman said.
"What we need is a separate agency whose 100 per cent focus is on ensuring our cyber protection.
"Mass spying and cyber protection are two separate issues. It is wrong for John Key to justify mass surveillance as being necessary for cyber security, it is not. "Cyber security is a real issue, and getting it right is critically important to New Zealand, however it is essentially unrelated to international spying which is the core function of the GCSB."
It was "easy to get the lines blurred" when the GCSB was responsible for both spying and cyber protection, he said.
"The Green Party has always said that a separate agency, free from spying, should be in charge of New Zealand's cyber protection efforts," he said.
"In Government we will undertake a wide ranging independent inquiry into New Zealand's intelligence agencies, and we will specifically look at separating cyber protection from the GCSB."
Next article: He's Dotcom's little henchman: PM attacks journalist's spy claims
