National leader Simon Bridges has ruled out any post-election deal with NZ First - the first big move of his year.
Bridges said today it was clear that a vote for NZ First was a vote for Labour and the Greens - and he wanted voters to have certainty when they cast their votes.
"I don't believe we can work with NZ First and have a constructive, trusting relationship," he told reporters in Hawke's Bay.
"Our decisions will be about what's best for New Zealanders, not what's best for NZ First."
It was the first thing decided at the party's caucus's retreat in Havelock North, Bridges said.
He said NZ First was "tied at the hip" with Prime Minister Jacinda Ardern.
"I can't trust New Zealand First."
NZ First leader Winston Peters responded by saying National's decision showed Bridges has "got a lot to learn about politics".
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Bridges said National was open to working with the Act Party - an indication that the Epsom electorate deal would be back on.
Bridges had said last year that he would set out his preferred options early this year.
It echoes former PM John Key's move in 2008 and 2011 to rule out NZ First - and in 2008 it was one of the factors that got NZ First booted out of Parliament.
National MPs arriving at today's caucus retreat in Havelock North were asked by media to describe Winston Peters, and used words including "crafty," "wily" and "done his dash".
Speaking to reporters this afternoon, Bridges said NZ First are "tied at the hip with Ardern".
He said New Zealanders were sick of the "charade" with NZ First and wanted to give Kiwis a clear choice before the election.
"National won't work with New Zealand First after the election, full stop."
That's even if NZ First again has the balance of power after September 19.
He said National would rather be in opposition for another three years, than work with NZ First.
Bridges said the National Party thought it was the right thing to do – many, he said, were relieved this was the caucus' decision.
"I think this will be the closest MMP election ever – but I'm confident National will form a government.
"We're playing to win, and I think we will."
He would not be drawn as to whether his move was designed to kill the NZ First party.
If NZ First does not get above 5 per cent, and fails to win an electorate seat, the party will not get back into Parliament.
There was nothing that NZ First, or its leader Winston Peters, could do to change his mind on this, Bridges said.
One of the major reasons Bridges won't work with NZ First is due to the fact that last year, while Peters was in coalition talks with National, he was in the process of suing National.
He said he has received "many emails a day" asking National to rule out working with NZ First.
Peters said National's decision narrowed their options and "can be the worst strategic move you will ever make".
'Having been in politics a long time, and a member of the National Party for over 25 years, the one thing New Zealand First is confident about is that if voters deliver that possibility, and if Mr Bridges doesn't pick up the phone, someone else within his caucus will do it for him," Peters said.
"He has also demonstrated he has no insight into what a unified caucus looks like.
"As Douglas McArthur said, there'll come a time soon when he'll when want to see me much more than I want to see him."