Key Points:

Prime Minister Helen Clark and National Party leader John Key have refused to share the stage with other party leaders in an election campaign TV debate.

They say only their head to head debates really matter because one or the other will lead the next government.

TV3 and TVNZ wanted the leaders of all eight parties represented in Parliament to take part in an MMP debate, which has happened in previous campaigns.

But the two leaders refused, and did not change their minds when both networks asked them to reconsider.

"There are two candidates for prime minister - I'm one and the Leader of the Opposition is the other. That's why the head to head debates are so important," Helen Clark said on TV3 News. Mr Key held the same view.

"There are only two people who can lead the next government - Helen Clark and myself. It's a very important opportunity for New Zealanders to compare and contrast."

They will appear in two leaders debates during the campaign.

Neither would say which had instigated the agreement that neither would appear with the other leaders.

Political scientist Jon Johansson said they were arrogant, and Act leader Rodney Hide said it was the "old club" arrangement where Labour and National stitched up a deal between themselves.

In 2005 there was a row when TV3 tried to exclude United Future leader Peter Dunne and Progressive Party leader Jim Anderton from the MMP debate.

The network said eight was too many, but Mr Dunne and Mr Anderton went to court and TV3 had to let them take part.