New Zealanders will be "shocked" by revelations of spying, investigative journalist Nicky Hager predicts ahead of the release of leaked documents.

New Zealand's role in the spying network led by the United States will be revealed on Thursday morning when a selection of documents is set to be published online.

Hager collaborated with news site The Intercept, and newspapers the Sunday Star-Times, the New Zealand Herald, and the Herald on Sunday, to publish stories based on files taken by whistleblower Edward Snowden from the United States National Security Agency.

Hager said on Wednesday that he and a colleague had been mining their way through the data for months.

"The general theme is that the New Zealand Government has been saying that the GCSB is used for major things like ISIS and terrorism threats. New Zealanders will be shocked to find what it is actually being used for."

He said it was "so much more than spying."

Hager added that so far New Zealanders had been watching the leaks regarding other countries.

"Now we will find out what New Zealand is doing."

Earlier, Prime Minister John Key urged New Zealanders to dismiss claims about spying on foreign allies, saying he can "guarantee" they will be wrong.

Journalist Glenn Greenwald last year said the Snowden documents would show New Zealand spied on its Pacific neighbours and other Western democracies.

READ MORE: Q&A - Spying and NZ

FROM THE ARCHIVES: NZ spied on Pacific neighbours - Greenwald

Today Key went on the attack, giving "very strong advice" for New Zealanders not to believe Hager, whose Dirty Politics book was a major theme of the September election.

That book alleged that National used a strategy of making Key the friendly face of the government while using right-wing blogs, most notably Whale Oil, to attack opponents. It was based on correspondence hacked from Whale Oil blogger Cameron Slater's computers.

Key claimed the material in Dirty Politics was wrong, and the same would be the case this time.

"Last time he came out with all this stuff, he was categorically wrong, he'll be wrong this time as well, because information changes, we review things all the time, different actions are taken," Key said this afternoon.

It had never been in doubt that New Zealand gathered and shared intelligence, he said.

"What a bizarre time to be coming out making the case that New Zealand either gathers and shares information or gets information from other intelligence agencies.

"Well, of course we do, and we do that to keep New Zealanders safe. We're in the situation where we've got ISIL [Islamic State] reaching out to cause harm to New Zealanders, I think New Zealanders would expect me to share information," Key said.

"My very strong advice to New Zealanders is [to] discount massively everything you hear from Nicky Hager. He was wrong last time, he's wrong this time, his interests are his own self-serving interests, not the rest of the country."

Key did not confirm or deny that New Zealand's spy agencies were spying in the Pacific.

"I'm not going into who we gather information from, or why, but I can tell you we do gather information, we have over successive governments across a range of different places, but we do that for really, really good reasons."

"We don't do that loosely or randomly, and actually, those situations change dramatically."

FIVE EYES

For more than a year Key has been managing expectations about what the Snowden documents might contain about New Zealand.

Diplomatic sources across the Pacific said on Wednesday he had largely succeeded. However, separate diplomatic sources say publication could create tension in Fiji and Tonga.

"Fiji is a wildcard in these things," a diplomatic source said.

Newly democratic Prime Minister Frank Bainimarama has a track record of expelling diplomats that anger him – particularly those from New Zealand.

Documents leaked by Snowden in 2013 revealed the Five Eyes countries - the United States, Canada, Britain, Australia and New Zealand - had been spying on one another's citizens, sharing the information to circumvent domestic restrictions on spying on their own citizens.

The release of Snowden's documents has been ongoing, with some in October 2014 suggesting the Government Communications Security Bureau used New Zealand embassies to snoop on foreign powers.

Other snippets relating to New Zealand also suggested the NSA could be using the GCSB, as a Five Eyes partner, to hack into other nations' systems.

One classification refers to "second party partner assisted network infiltration operations."

As far back as November 2013, Key said he thought it was "highly likely" that Snowden had information about New Zealand's spying activities.

At the time, Key said he was "comfortable with the way our agencies operate and I'm comfortable they're not breaking the law".

Documents already released at that point revealed New Zealand's role in assisting the United States in spying on other countries as part of the Five Eyes networks.

Less than a week before the 2014 general election, Snowden appeared by videolink at a "Moment of Truth" event at the Auckland Town Hall.

Snowden said he regularly viewed the private communications of New Zealanders in his role as an NSA analyst.

The content and metadata was available to him through the programme known as X-Keyscore, which Kiwis were subject to, Snowden said.

Snowden also said at the "Moment of Truth" there were NSA facilities in New Zealand that the GCSB and the prime minister were aware of.

Key dismissed Snowden's assertions that the NSA is operating in New Zealand, but refused to discuss X-Keyscore.

Greenwald claimed then that New Zealand had spied on friendly countries for the United States.

Greenwald compared New Zealand's "harmless" image to that of Canada, which was revealed in another Snowden leak to have spied on the Brazilian Ministry of Mines and Energy.

Canada's equivalent to the GCSB had set up spying sites in 20 "high-priority" countries.

New Zealand's value to the Five Eyes alliance was its relationship with other countries the United States may not have access to, Greenwald said.

The journalist warned of diplomatic implications following further Snowden leaks, as he said a lot of people would be surprised about the Western democracies and allied countries New Zealand had spied on.

Key dismissed Greenwald as a "loser".

The night before the "Moment of Truth" event, Key declassified intelligence documents which he said proved the GCSB was looking at implementing a cable tap programme on the Southern Cross cable.

The plan would have allowed surveillance of all traffic on the cable, but Key stopped it before it was implemented.

Instead of "Project Speargun" being put in place, the GCSB got "Cortex," which intercepted email traffic to particular companies, not mass wholesale surveillance.