Prime Minister John Key has accused Labour leader David Shearer of sneaking up to his office for secret talks about new spying laws.

The extraordinary exchange occurred during Parliament's Question Time this afternoon as debate continues over the Government Communications Security Bureau (GCSB) bill.

Shearer asked Key if he or any of his staff had contacted Labour as the Government worked to gain cross-party support for the proposed legislation.

An incredulous Key responded: "I cannot believe the member is asking that question."

He then went on to describe an encounter after an Intelligence and Security Committee meeting.

"I asked Mr Shearer if he would like to come to my office to have a discussion," Key said.

"We sat down and had about a 30 minute discussion where Mr Shearer said 'keep this confidential. If you come out and say we've done it that won't look good and I don't want you shouting it about the house'."

Key added: "On numerous occasions we reached out and at one point - and if the member really wants to get down and dirty, members of the Labour party said they didn't understand the law."

Shearer did not deny the meeting - but asked if Key was claiming that he initiated contact.

"It was my request, he should remember that," Shearer insisted.

Key blasted back: "I'm afraid the member is wrong. I went up to the member after the ISC and said quote unquote 'do you want to come to my office.' To which the member said 'yes'. I said we'd probably take the stairs to avoid the other guys.

"And we actually waited 'til the other members, and in particular Dr Norman to leave so that... he didn't see you coming up to my office."

The exchange was greeted with uproar from fellow MPs. Shearer accused Key of telling him "that he didn't really care" about securing Labour party support, because he already "had Peter Dunne over the line".

Dunne withdrew his support for the bill, before performing a u-turn after winning some concessions.

Key said he didn't "exactly" say that.

"What I said was we had the numbers to pass the legislation and I knew that if, for political reasons, he [Shearer] was just going to carry on the way he was going because [deputy] Grant Robertson was calling the shots, fair enough."

Key also accused Shearer of not understanding the new laws and acting like "a bush lawyer".

The bill was expected to pass into law tomorrow with a single-vote majority.

SHEARER FIGHTS BACK

Shearer emerged from the House to defend the July meeting, and denied it was concealed from other parties.

"We went up to his office and had a chat about the progressing of the bill ... it was simply about the Labour Party talking about the National Party," he said.

Shearer said he wanted the details of the discussion kept private.

"But the Prime Minister, for example, this morning said that Labour had made no approach to him and that was patently not true," he said.

"I actually sat down with him twice, both times after [ISC] meetings.

"The trouble was a little while later I saw some of the elements of that meeting actually coming out and Mr Key talking about it in public meetings. It annoyed me that I had actually asked to keep that quiet and confidential."

Shearer insisted "who approached who" was important - he instigated the meeting and Key had his "facts round the wrong was".

"This is the Government's bill, the Government did not do anything to try and initiate a sit-down with other parties in order to get broader consensus across the House," he said.

Labour had proposed a sunset clause which would allow the GCSB to resume surveillance work, on behalf of other law enforcement agencies, while a review was undertaken, he said.

However, Key rejected it because he had already secured Dunne's backing.

"I don't care what Mr Key says, that's my memory of it," Shearer said.

ACT leader John Banks said the incident marked "the beginning of the end" of "hapless" Shearer's leadership and he was naive.

"He left himself wide open and got everything ... he got caught on the chin," he said.

Banks said it was "no secret" to him the meeting was being arranged.

LABOUR IDEAS 'NOT NEEDED'

Prime Minister John Key said today that Labour's idea to add specific protections for the content of emails was not needed.

He said it was already provided by the interaction of three clauses in the Government Communications Security Bill, which was expected to pass into law tomorrow.

He confirmed that he would make clear his intent, including that email content would not be accessed without a second warrant, in his third-reading speech tomorrow.

That would give judges interpreting the law in future a clear steer on the Government's intentions.

Key said the change Labour wanted to make was to the first part of the law, and that had already been voted on in committee stages by Parliament.

Labour leader David Shearer said he would today move a motion asking Parliament to go back to part one to vote on his amendment.

It would make it explicit that a new warrant would be needed to access the content of a New Zealander's communications, including emails, under the cyber-protection measures.

The move would need the backing of UnitedFuture leader Peter Dunne, who today indicated he was unlikely to vote with Labour.

He said he had not reached a final position but was "not inclined to support" late amendments.

Key said Labour's ploy showed it did not understand the bill.

When it was last before the House, the Opposition strung out debate to slow its passage when all they had to do was put up the amendment that they wanted to introduce now, he said.

Key said the opposition to the bill was political and most of the protesters at a meeting in Auckland last night were aligned to the Left.

He said he and Government Communications Security Bureau director Ian Fletcher would resign if the spies undertook mass surveillance of New Zealanders.

Shearer said: "What we're trying to do is to make sure that what John Key has said can actually become part of the law ... so we will be able to look at that and be able to say that nothing can go through unless that is actually given the approval."

The change was one thing that would "make a bad bill a little bit better", he said.