New National Party leader Judith Collins is already targeting Prime Minister Jacinda Ardern - and ensuring she won't get away with any "nonsense" - but one of her first priorities last night was to take stock after one of the most extraordinary days in New Zealand politics.

"I think it might be time for a drink," said Collins, as she concluded her first press conference as National leader, a few seconds before 10pm on Tuesday.

It ended a bruising day for the party, in which Todd Muller quit as leader at 7.30am, MPs made their way to Wellington for an emergency caucus meeting, and Collins emerged victorious as the leader after the contested leadership race against Mark Mitchell.

National MPs took two hours late last night to decide she should be their new leader, with Gerry Brownlee elected as her deputy.

It was the "third time lucky", said Collins - this was her third run at the leadership.

The National Party hasn't had a female leader for 19 years, with Collins the second woman to have the job after Jenny Shipley.

Collins told Newstalk ZB's Mike Yardley today that she was feeling "very hopeful".

"We will do our very best - we didn't see it coming. I'm still getting over the excitement of it all."

She said she had not spoken to Muller since his announcement but added: "I have heard from one of his close friends that he and his wife were delighted with the results yesterday."

Collins last night said she would unite her caucus after their second leadership change in less than two months.

"I think it's really important we all have a common goal and the common goal is to get rid of the current government and put in place a far better government focused on the people of this country," she said at a press conference and standing in front of her caucus.

About her taking on Ardern as her adversary, Collins said she'd always respected the Prime Minister and thought her party had underestimated her for years.

"I think it's important to give credit where it's due.

"But at the same time there is no chance at all that I'm going to let Prime Minister Jacinda Ardern get away with any nonsense when it comes to the economy and doing her job.

"We will hold her to account."

Asked what she had over Ardern, she said: "Experience, toughness and the ability to make tough decisions".

When asked if she could shake the nickname "Crusher", she said: "I am hoping that the National Party will collectively crush the other lot, come September 19."

She said she never worried about what happened in the past. "We're just going to have the most fun as we take back the country."

She added: "We're a serious party, with serious policies."

Collins confirmed this morning that Paula Bennett was sticking with her decision to leave politics. She had yet to talk to Amy Adams, who had planned to leave politics but changed her mind when Muller became leader. Collins said she got on well with Adams.

Collins told Newstalk ZB she was not planning a major reshuffle of MPs' portfolios, other than assigning the roles she and Brownlee held. "We don't need a big reshuffle, we just need to accommodate the changes."

Asked if Simon Bridges would continue with the foreign affairs role she said she did not see why not. She expected Bridges would be "more occupied" with the leadership change.

"I don't think there's a great need for any massive change," adding there was no need for any overhaul of policies.

She would be reassessing the upcoming infrastructure speech as she sat down to examine her diary. The gangs were still a focus of the National agenda.

"We have a country that is about to go through very difficult economic times and people need to have a feeling of certainty that things are going to get better quicker."

People needed to be realistic but "we have a way through", she said.

On aligning with Winston Peters, she said: "We work with New Zealand First now on various pieces of legislation. I don't think that's going to be the big issue and reasonably comfortable that it won't be the same issue as last time."

She said after her years in Parliament she had learned a valuable lesson in longevity and aspiration. "Never give up. Just keep going," she said.

"I thought very quickly about whether putting my hand up in these circumstances and didn't come into Parliament to be mild. I can make a difference."