Now Judith Collins is at the helm of the National Party, she'll come face-to-face with Prime Minister Jacinda Ardern in September's General Election.
The leaders' debates in the run-up to the election will provide a more in-depth look at major party policies, but where do the pair personally stand on other issues?
Here's a look at Ardern's and Collins' stances on same-sex marriage, abortion, euthanasia and cannabis.
Both Collins and Ardern voted in favour of a Bill in 2013 allowing same-sex marriage.
Ardern said at the time the answer for her "had always been yes" when voting for marriage equality.
"The fact that, whether this bill passes or not, my rights and my privileges as a heterosexual woman will continue. I can choose to marry, or not. I can choose to enter a civil union, or not," she said.
"I am not the one who has experienced limitations on my rights or freedoms, and yet here I am, privileged enough to have a vote to determine whether that right should be extended to others."
Collins told GayNZ.com she had no problem with same-sex marriage or allowing same-sex couples to legally adopt children.
"Frankly it would be really nice if we could look at people as human beings rather than always saying 'you can't do that because you're gay' or whatever."
Collins voted against the 2004 Bill creating civil unions, "not because of any sort of homophobic views" but because it created "a parallel form of marriage".
"This Bill is a sop to gay couples, in which they are being told that they can have second best. That is not good enough. Either people are entitled to look at themselves as married, and to get married, or they are not," she said during the first reading.
She also voted for the Marriage (Gender Clarification) Amendment Bill in 2005, which would have defined marriage as only between a man and a woman. This failed in its first reading.
Both Collins and Ardern also voted in favour of the Abortion Legislation Bill that removed abortion from the Crimes Act.
Ardern said she was asked during the 2017 election campaign whether there needed to be abortion law reform, and she had an instinctive answer.
"At that point, for me it was a simple, intuitive reaction: yes. Abortion should not be a crime in this country," she said during the Bill's first reading in 2019.
"I am one who takes a very simple perspective: that, in spite of those personal views, who am I, who is this House, to determine anyone else's reproductive rights? So, in my view, you can have a deeply held personal view… and I will always defend your right to hold that view, but I will draw a line when holding that view then impedes on the rights of others."
Collins said at the time she supported the Bill not because she was pro-abortion, but because she understands the reality of life for many women.
"This is not a pro-abortion move, in my opinion. This is about a reality check. I would personally rather that abortions, if they must occur, occur very early on in a pregnancy. I would much rather have that happen," she said during the first reading.
"These things happen. We women have dealt with it for generations, for hundreds of years. It is not a nice place for anyone to be with any pregnancy that is unwanted. But I do think we have to understand the reality. We have to support women when they're going through this, and we have to have abortions, if they must be, early - as early as possible - and with the least trauma as possible."
Similarly with abortion reform, Ardern said she supported the 2019 End of Life Choice Bill because she doesn't want to stop others being able to access it.
"My view is the best way that I can allow people to make their own decisions is actually giving them access to that choice. At the moment there is no access to that choice because of the law.
"I would like others to have their own choice and that means voting in favour of the Bill."
Despite Collins voting against the 2003 Death with Dignity Bill and also the first reading of the End of Life Choice Bill in 2017, she voted in favour of it in its final vote in 2019.
During a debate on the proposed assisted dying law last year, she held back tears as she gave an emotional speech about her father, who died from terminal bone cancer.
Collins said her father was in "terrible pain" in hospital and needed morphine. He was given the morphine and "died without losing his dignity", she said.
"I have always been opposed to euthanasia as of right on the basis that people like my dad got to essentially tell everybody when they wanted to go - and I thought that was available to everybody... it's not available to everybody," she said.
"It's not available for people like my dad who happened to be in the wrong place at the wrong time and without a family saying, 'You give my dad everything he needs'."
Collins said she used to be opposed to assisted dying, but now believes giving people the choice to die with dignity is the right thing to do.
"I am on the right side now - everybody deserves some dignity in their lives. I would do it again, it's the right thing to do, and it preserved his dignity."
Ardern has remained tight-lipped on how she'll vote in the upcoming cannabis referendum.
In 2018, the Government passed the Misuse of Drugs (Medicinal Cannabis) Amendment Bill, which allows terminally ill people to possess and use cannabis.
But although she's never confirmed or denied her support, when asked by NZME in 2019 if she's ever used cannabis, she gave a coy response.
"I've openly acknowledged in the many times that I've been questioned as a Member of Parliament - and which most Members of Parliament would be asked exactly the same thing - I was raised Mormon and then I was not Mormon. I let other people determine what that means."
Collins says she's never smoked marijuana, but has often shared research on cannabis.
In 2009 Collins voted against Metiria Turei's Misuse of Drugs (Medicinal Cannabis) Amendment Bill, which would have allowed cannabis to be used for medicinal purposes.
In 2018, National said it supported the concept of medicinal cannabis and it introduced an alternative Bill at the same time as the Green Party's Misuse of Drugs (Medicinal Cannabis) Amendment Bill 2018 progressed through Parliament. National voted against this.
Appearing on The AM Show in October 2018, Collins declined to say whether she'd be likely to vote in favour of legalising recreational use of marijuana, although she acknowledged that personal use will "probably" end up legalised soon.
"I can see that there's not a lot of point, and police certainly have stopped prosecuting people for possession... There are 70 percent fewer prosecutions than there were 20 years ago.
"The problem is once you say something's decriminalised or it's fine and we're going to have it sold in pharmacies and shops, people think it's okay. We say that with so-called synthetic cannabis - it was actually completely able to be sold in various places, and it just took off.
"We're parents, and we don't want to send messages to our kids and other people's kids that something's just perfectly fine. Actually, it's not perfectly fine."
But in an interview on Wednesday with The Project, she said she didn't want marijuana legalised.
"I think that a lot of people are very happy with medicinal cannabis, but not so happy with recreational use," she said.