Judith Collins is downplaying her claim that her leadership rival Amy Adams broke the caucus rules when she launched her campaign with an entourage of four National MPs.

Adams had the public support of Nikki Kaye, Chris Bishop, Tim Macindoe and Maggie Barry when she announced her bid for the National Party leadership on Wednesday.

Asked about the endorsement of fellow MPs, Collins suggested yesterday that the public backing of Adams had gone against the wishes of the caucus and 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," Collins said.

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But the party's chief whip Jami-Lee Ross soon said 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."

She added that the MPs supporting her had made a decision to do so publicly, and she had not asked them to.

This morning Collins downplayed the episode, saying she had not alleged that the Adams entourage was against the wishes of caucus.

"I didn't actually complain. It was one of those things where I was asked a question about why I didn't have people with [me] ... I simply answered the question," she told The AM Show.

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"Now there's an allegation I haven't really made at all."

Collins announced her run for the leadership on Twitter, "so it would have been a bit weird" to have had the public backing of fellow MPs.

She said it was not a big deal.

"People can get all fragile about these things, but I just think it's part of a bit of rough and tumble."

She also confirmed that it will be a preferential vote to decide the leadership, meaning MPs will have a secret ballot, and the candidate with the lowest support will drop out of the race after each round.

MPs will then vote again until one candidate reaches a majority of at least 29 votes.

The system means that the candidate with the most votes in the first round will not necessarily win, and it was important for candidates to lobby MPs for second- and third-round votes.

Yesterday Jonathan Coleman ruled himself out of the race, meaning that Collins, Adams and Simon Bridges remain the only people in the race.

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

Mitchell is in Australia supporting his daughter until Monday and is not expecting to make an announcement until he is back in New Zealand.

Collins would not say how much support she had, but believed she had a strong chance of winning.

"We'll have to wait and see. I'll tell you what, though - it's so much fun."