National's leadership contest is intensifying, as the party whip shoots down Judith Collins and her claim that rival Amy Adams broke the rules when MPs publicly backed her.

Yesterday during her launch, National MPs Nikki Kaye, Chris Bishop, Tim Macindoe and Maggie Barry stood alongside Adams in a public show of support.

Collins claimed today that the display had broken an agreement in caucus, and the wishes of retiring leader Bill English.

"The fact is that the caucus agreed that we would not come out and be counted in those sorts of things until after the vote. That was the advice of Bill English, and I'm sticking by it."


But National's chief whip, Jami-Lee Ross, released a statement a few hours later, saying Collins was wrong.

"While discussion took place, no caucus decision was made regarding MPs taking a public position in endorsing a candidate for leader of the National Party," Ross said.

Adams responded by seeking the high ground, saying she was focused on her campaign.

"I've broken no rules whatsoever. MPs can stand wherever they like. I'm running my campaign in an incredibly clean and ethical manner, and my integrity is very important to me.

"It's for Judith to decide how she wants to run her campaign."

Earlier today Jonathan Coleman ruled himself out of the race, meaning that Collins, Adams and Simon Bridges remain the three people currently in the race.

Steven Joyce and Mark Mitchell are still deciding whether to run.

Coleman said he decided the best person to lead the party into the 2020 election was one of the three current candidates.

"I didn't go out canvassing the numbers. In the end I looked at who would be best to lead the party, and it's one of those three."

Collins called on party members to let their MPs​ know their thoughts on the leadership, and said that some backbench MPs should have already been promoted.

"We have, in our new intake and in 2014 and also 2011, some really good people who feel, quite rightly, they haven't been given a chance, and I think that in Opposition, that's the time to do it.

"But it should always be about merit ... it's about who can do the job. That's my bottom line for anything."

Collins had earlier put a line in the sand at 35 per cent support in the polls, but Adams declined to match it, saying it was up to Collins to set her own bottom lines.