Judith Collins comes out swinging - at Prime Minister Jacinda Ardern

National MP Judith Collins has launched her leadership campaign with a swing at Prime Minister Jacinda Ardern, saying Ardern talks about herself "and her feelings" too much.

In an interview with the NZ Herald after announcing her bid for the leadership, Collins said she recognised Ardern was "a formidable opponent."

"And people have underestimated her at their peril."

"But the main thing with Jacinda Ardern, it seems to me, is that it's all about her. What we need to do is to be very focused on the people, on New Zealanders and what is being delivered or not delivered to them."

"I think she talks a lot about herself and her feelings. I think what she really should be doing is focused on the people."

She said others might have a problem taking on Ardern out of concern of a backlash because Ardern was a young woman and pregnant.

"I have been pregnant, running a law firm and studying as well. As a young mum I understand exactly how tough it is to do that role and undertake a very difficult and tough role. But she understands that too.

"That is the way she has chosen to do that and I always support people when they want to have children. But that's not the point. That is not the role she's asked New Zealanders to support her for. She has asked them to make her and to keep her as Prime Minister of New Zealand. And that is the role I would hold her to account for."

Collins also put in a pitch to her caucus colleagues, saying this was the first chance in more than a decade to have a "real" say on the leadership.

"This is their chance. They can either do the status quo and more of the same. Or they can understand this is a real war, a real battle we are in and the National Party, if it wants to really be in there in 2020, we are going to have to do things a bit differently."

She said she did not intend to withdraw from the ballot, even if became clear she would not win. Nor would she leave Parliament if she lost, saying she was voted in by Papakura and that meant a lot.

"There's no way I'm going anywhere."

Collins said whoever got the leadership would be in for a tough time ahead as it was hard to counter the effect of things like Budgets "and dishing out money and doing things they think will buy them votes."

She said what was needed was somebody who had real experience in Opposition and been effective at it.

"We need to have very decisiveness and strong leadership in Opposition, and that's something I can bring."

Collins said she had approached people to advise them she would be happy for them to be her deputy and some had approached her.

However, she said it was a matter for caucus.

Paula Bennett has said she wants to stay in the role, so any contenders would have to challenge her. Collins said she would work with whoever caucus told her she'd have to.