Russel Norman's resignation as Green Party co-leader has been hailed by co-leader Metiria Turei as a vote of confidence in her leadership, the caucus and the party.
He will stand down at the party's AGM in May after nine years in the post.
Norman said his present plan was to stay on in Parliament after the leadership change but he would assess other options.
"I concluded that after nearly a decade, it is a good time to find a new challenge for myself and to spend more time with my family, and now is also a good time for new leadership for the party."
He announced his decision at a press conference at Parliament this morning, which the Green caucus attended.
He made the decision over summer during which time he and partner Katya Paquin had their third child. They now have three children aged under four.
He said he wouldn't be leaving if the Green Party if it was in a weak position. "I'm leaving for my own reasons but the Green Party is in a strong position so that really gives me confidence."
Dr Norman said he had wanted a good summer break to think about his future. "It's something I have thought about for a long time, about what are the kind of challenges I want to do in my life. I'm 47 years old so I need to look at where I want to and what I want to do and I want to spend a bit more time with my kids. So summer was a good time to think about it. It was a great summer."
'Sad to lose my fellow captain'
Australian-born Dr Norman was elected co-leader in 2006 after the sudden death of Rod Donald in 2005. Ms Turei became co-leader in 2008 when Jeanette Fitzsimons stood down.
Dr Norman carved a distinct role for himself as the party's economic spokesman and was often praised mischievously by National as the true leader of the Opposition.
Ms Turei said she trusted Dr Norman's judgment.
"Russel's decision is a vote of confidence in me, in our caucus and in our party and for that I am enormously proud to stay as co-leader with that vote of confidence, that we can continue to build and strengthen our work and increase our work and our support."
Mrs Turei said it was "sad to lose my fellow captain".
"But I have the utmost respect for Russel's decision...I am frankly in awe of what Russel has done, given that he has been raising a young family at the same time."
With emotion in her voice, she said: "Russel stepped up when we needed him most. Her has led us with passion and with drive since Rod Donald's death. He has been amazing to work with, one of New Zealand's foremost thinkers on delivering thought leadership on smart green economics in this country."
The party was returned with 14 MPs after the September election, the same number as the previous term, but again the third largest party in Parliament. Its percentage of the vote decreased but actual numbers voting Green increased.
The Green constitution requires a male and female co-leader.
The new male co-leader leader will be elected by the party conference following a primary-style contest, similar to the one the Labour Party ran - where candidates will make their pitch to member and the media may be present.
The front runner is likely to be West Coast-based MP Kevin Hague.
New Green MP James Shaw and Gareth Hughes could also be contenders but the post will be open to any male financial member of the Green Party.
Dr Norman was initially co-leader without being an MP but acknowledged that it was difficult to be co-leader outside Parliament.
Nominations for the new co-leadership position will open on March 20 and close on April 17.
On social media, the reaction to Dr Norman's resignation was one of sympathy:
Russel Norman's full speech to caucus
I am announcing today that I will not be standing for Co-leader of the Green Party at our AGM in May.
This is my ninth year as Co-leader and I think it's time for a change.
This is something I have considered for some time and over the summer break I have had the space to think hard about my future.
I concluded that after nearly a decade, it is a good time to find a new challenge for myself, and to spend more time with my family, and now is also a good time for new leadership for the party.
My replacement will start from a strengthened base and will have a full parliamentary term to establish himself in the role and take the Greens into government in 2017.
The time is right.
I have always held the view that no organisation should be reliant on any one individual, or indeed two.
I am proud of my contribution to the Green Party and the Green movement, but know that others can pick up and build on this work and take our party to even greater success.
One of the hallmarks of the Greens has been our successful leadership renewal.
When I became male Co-leader in 2006, after the tragic death of Rod Donald, many commentators chimed the death knell of the Greens. How wrong they were.
In my nine years as co-leader we have more than doubled our electoral support to over 10 per cent.
I am personally gratified to have been part of a team that led our party to its two most successful election results ever, the only time under MMP that a party other than Labour and National have received more than 10 percent of the vote two elections, in a row.
The Greens are now unquestionably the third party in New Zealand politics and are an important and influential part of our political landscape. I am very proud of that achievement.
Our electoral success has been built on developing policy that is both principled and realistic, a strengthened party organisation, and a highly professional and effective parliamentary structure.
Our membership has never been higher and our branches never more active. That forward momentum will continue.
It has been a pleasure to work alongside a smart and passionate caucus and dedicated staff who are the best in the business.
We have put forward positive smart solutions to the issues facing our country, while leading the opposition and holding the Government to account.
Our policy work in the last election is a blueprint for modern progressive green government and I am proud to have played a key role in shaping that.
In particular, as lead spokesperson on economics and the environment, I have relished the opportunity to develop policy that is both good for the economy and good for the environment. It has been inspirational to see, in spite of a disinterested New Zealand Government, that there are so many great businesses and organisations taking up the opportunities of sustainable economics in New Zealand and around the world.
Those countries and businesses that embrace the opportunities of sustainability will dominate the twenty first century.
The Greens are focused on the future and not simply managing a status quo built on the past.
That is the distinctive role that the Green Party plays in our democracy. We are thought leaders and catalysts, pushing ideas and solutions against the headwinds of fear and inertia.
We operate at the vanguard of change and it has been a pleasure to be at the helm for a time.
But as they say, all good things must come to an end.
I started as a party volunteer in 1997 and later, after my PhD was completed in 2001, I started work in the Green Parliamentary office.
I never imagined I would end up as Co-leader.
It has been a most rewarding job.
I want to take this opportunity to thank my colleagues in the Green Party for their unwavering support over my nine years as Co-leader.
In particular I want to thank Metiria. Together we have built a true political force that serves New Zealanders well and offers genuine choice and leadership on the issues that matter. I know that you will carry this work on with a new male Co-leader.
I also want to acknowledge the support of my family and in particular my partner Katya. I know that you have carried alot at home while I have been off at events up and down the country.
Being Co-leader while raising a young family has been an interesting experience, one that I feel has enriched my political work and strengthened my passion for building a better world for the next generation.
One of my political heroes Gough Whitlam once said 'a conservative government survives essentially by dampening expectations and subduing hopes. Conservatism is basically pessimistic, reformism is basically optimistic'.
I will leave this role optimistic. Optimistic for our party, our country and our world.
We will continue to reform with optimism.