National leader John Key says he is not overly concerned about whether the worm in last night's leaders debate was biased.

"I think people who have watched the debate will come away with their own thoughts about it. I was happy with the debate, I thought I had the opportunity to make critical points,'' he said today.

Mr Key said he was happy he had gone on the TV3 debate, moderated by John Campbell.

His comments follow criticism of the debate by National Party member and Kiwiblog founder David Farrar, who claimed at least three Labour voters and a Green voter were among the "wormers".

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But debate producer Keith Slater said the panel was carefully selected under strict criteria by a reputable company.

Mr Key is today on his last campaign visit to the South Island, spending the morning in Dunedin before heading to Christchurch this afternoon.

He stood by his criticisms during the debate of NZ First and Labour's apparent willingness to work with Winston Peters.

The prospect of the return of Mr Peters came to the fore during the debate with Labour leader Phil Goff.

If the unscientific worm is anything to go by, Mr Goff was the clear winner, but Mr Key struck a chord, distancing himself from the instability that Mr Peters could throw into the mix should the New Zealand First leader return to Parliament.

The worm was a group of 65 undecided voters reacting to the debate.

It was the final topic of the debate, MMP, that allowed Mr Key to paint a picture of a Labour-led "cocktail'' of parties including Mr Peters, who he said could not be trusted.

"It's comedy-central. There's no way anyone can argue Winston Peters will provide stability.''

This was the time in the debate where the worm clearly favoured Mr Key, and the worm fell away every time Mr Goff tried to defend Mr Peters.

Mr Goff has avoided saying whether he trusts Mr Peters, and tonight switched tactic to say he trusted him to follow through with any post-election agreement.

He countered Mr Key's claims of instability by referring to National's preferred coalition party, Act, which has no returning MPs seeking re-election and has a new leader.

"We're not about to rort the system by putting back in Parliament a party that less than 1 per cent of people want.''

After the debate both leaders were happy with the way it was conducted, the use of the worm and the subjects that were discussed.

Both declined to claim victory.

Mr Key and Mr Goff will square off for a final debate on TVNZ on Wednesday.