Prime Minister John Key is digging his heels in over spying claims - refusing to explain the difference between the terms "mass surveillance", "mass collection" and "full-take".

At issue is Key's promise to resign if the Government Communications Security Bureau (GCSB) was found to have undertaken mass surveillance of New Zealanders. Commonly this refers to monitoring the communications of whole, or substantial part, of a population.

Documents leaked by fugitive American whistleblower Edward Snowden show the GCSB was collecting enormous amounts of email, phone and social media communications in Pacific countries and passing it to the National Security Agency (NSA) in the United States.

Critics point out large numbers of Kiwis live, work and holiday in the Pacific Islands, and those in Niue and the Cook Islands hold New Zealand citizenship.

There is also confusion over whether the Government believes surveillance and collection refer to the same activity.

The Snowden documents refer to "full-take collection" by the GCSB in almost two dozen countries.

Journalists have repeatedly asked Key what he understands the terms to mean.

Key said mass surveillance of Kiwis by the GCSB would be illegal: "I don't even know what you mean by mass collection. I have no clue. It is not a term I have ever used. It is not something that sits in something I see."

However, this differs from his responses at a media conference on Monday.

Key was asked if he drew a distinction between mass surveillance and mass collection.

"I am sure the lawyers would tell you there is a difference," he said.

"But I am not going to critique all those different points."

He was then asked if his promise to resign extended to the mass collection of information.

"No, because in the end I was asked a very specific question, without re-creating history, and that was: are we conducting mass surveillance of New Zealanders? And the answer is: No," he said.

Asked about those comments, on Tuesday, Key said: "Everyone will have their own legal definition and I am not about to provide those. But I think it is pretty clear what we mean by mass surveillance, which means an entire population or at least big part thereof. Everyone, I think, understands what surveillance is, and that is against the law."

When asked to explain "full-take," Key told a journalist: "I have no idea what that means. You are just winding yourself into knots which are a complete waste of time."

The prime minister has repeatedly stressed that the GCSB can only target New Zealanders under a warrant. However, the law allows for some exceptions, one of which is when New Zealanders are accidentally caught in a surveillance net.

There are concerns this would capture the vast number of New Zealanders who spend time in the Pacific Islands.

When asked by reporters to clarify the legal position of those Kiwis, and what happened to their data if it was collected, Key said: "I am just not going to go into what we do, how we do it, where we particularly have targets of interests."

He twice told one reporter: "you are terribly confused" when she asked if incidental collection of data was allowed for under a warrant. When she questioned what happened to New Zealanders' communications collected by GCSB offshore, he said: "I'm not going to go into that."

Another journalist trying to distinguish between private communications and metadata was told: "I think again you are completely thrashing around the sort of random ideas that just don't make sense . . . there is no mystery stuff here. You have got a bunch of people who are conspiracy theorists and it's not my job to go out there and go through all the legal definitions."

Key said he would not ask the GCSB to clarify the terms, to contribute to public debate: "Because it is not my job to go through the legal definitions...it's not creating confusion," he responded.

Labour leader Andrew Little said Key must be clearer with New Zealanders about GCSB activities.

"It's like nicking someone's mail - you might not open it, but you've got their mail," he said.

"That's the intrusion on the person's privacy, it's the same here."