An already tumultuous New Zealand election campaign took another dramatic turn less than a week before polling day when the prime minister, John Key, responded angrily to claims by the American journalist Glenn Greenwald that he had been "deceiving the public" over assurances on spying.

Greenwald, who is visiting New Zealand at the invitation of the German internet entrepreneur Kim Dotcom, says he will produce documents provided by the NSA whistleblower Edward Snowden that prove the New Zealand government approved mass surveillance of its residents by the Government Communications Security Bureau (GCSB), New Zealand's equivalent of the NSA.

Dotcom, who is sought for extradition from New Zealand by the US on copyright charges relating to his now defunct Megaupload file-storage site, is hosting an event in Auckland on Monday called The Moment of Truth, which doubles as a rally for the Dotcom-founded Internet party.

Greenwald has promised to produce his evidence at the event, while Dotcom is pledging to show further links between Key and Hollywood relating to his own case. Adding to the spectacle, Julian Assange is expected to beam in via video link from the Ecuadorian embassy in London, while Dotcom has hinted that Snowden may also appear on the big screen from Moscow.

In media interviews, Key has repeatedly dismissed Greenwald as "Dotcom's little henchman".

Speaking on TVNZ's Q+A programme, he acknowledged that the government had in 2012 considered a "mass cyber-protection" proposal, which he said was "really a Norton antivirus at a very high level", but rejected it. Greenwald, he argued, would therefore have seen incomplete material.

"This is what happens when you hack in to illegal information, when you wander down to New Zealand six days before an election trying to do Dotcom's bidding – what happens is you get half the story," said Key.

He said he was ready to declassify secret documents to support his argument. "There is no ambiguity here. There is no and there never has been any mass surveillance."

Greenwald responded by saying: "I absolutely stand by everything I've said." He told 3 News: "They did far more than look at the idea; they adopted the idea and took steps to make it a reality."

He added: "I've done reporting of surveillance all over the world and a lot of governments haven't liked what I've said, but I've never seen a head of government lose their dignity and get down in the mud and start chucking names to discredit the journalist in order to discredit the journalism."

Greenwald, a former Guardian journalist, has authored hundreds of stories based on leaks from Edward Snowden exposing the breadth of surveillance undertaken by the NSA and its partners in the Five-Eyes alliance, which includes New Zealand. The work was recognised with a Pulitzer prize in April 2014.

Even before the spying controversy, the Key campaign was bedevilled by scandal, with a senior cabinet minister forced to resign following the publication of a book based on hacked emails that revealed links between the ruling centre-right National party and an attack-blogger.

Key and his party have so far emerged mostly unscathed in the polls, however, and remain favourite to win a third term next Saturday.