Prime Minister John Key this morning said he would not be offering a retraction or apology to cameraman Bradley Ambrose.

On Friday, lawyers for Mr Ambrose wrote to representatives of Mr Key, senior minister Steven Joyce and the National Party, asking for an apology for what they claimed were "highly defamatory" statements about Mr Ambrose.

On Tuesday, the High Court will hear arguments on whether Mr Ambrose illegally recorded a conversation between Mr Key and Act candidate John Banks.

Mr Key said he still believes the conversation was private but he would abide by the court ruling.

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"It would be fair to say we wouldn't have taken the stance that we did unless we believed that we were right, which is that it was a private matter."

Speaking at Cox's Bay Park in Auckland, where he was opening the Westmere School triathlon, Mr Key also warned that Winston Peters holding the balance of power after the election would be a recipe for government instability.

Mr Peters, the leader of New Zealand First, has said he will offer confidence and supply votes on an issue-by-issue basis.

This morning Mr Key said that was a "very serious issue".

"Winston Peters is saying to New Zealand that on every budget, on every issue, there could be a general election.

"How can New Zealand govern itself over the next three years - which is likely to be a volatile period in the world economy - when at any stage the whole Government could be brought down by Winston Peters?"

Mr Key said that if he had the numbers to form a government after the election, he could provide a stable government

"Phil Goff cannot because the only way Phil Goff can really get there is with The Greens, New Zealand First, Maori and Mana parties," Mr Key said.

"That's not a recipe for stable government."

Mr Key was mobbed by schoolchildren as he opened the triathlon and signed hundreds of t-shirts.

One pupil remarked: "I'm never going to clean my t-shirt ever again."

Other children squealed at John Key but when asked what he did, did not know.