Green Party co-leader Rod Donald has died at the age of 48 leaving shocked colleagues and friends reeling at the news.
A Green Party spokeswoman said Mr Donald had died of a heart attack at his home in Christchurch.
His partner Nicola Shirlaw asked through a party spokeswoman that she and their three daughters Holly, Emma and Zoe be left to grieve alone.
Mr Donald was to be sworn in with other MPs at the opening of Parliament tomorrow but his seat will remain empty.
Prime Minister Helen Clark led a string of tributes to Mr Donald saying she was "shocked" and "deeply saddened".
"Rod has been very honourable to deal with. He always impressed me with both the strength of his convictions and his ability to work within the parliamentary system to resolve issues," Ms Clark said.
"Rod Donald was unfailingly pleasant, likeable, and always looking for a constructive way forward."
Miss Clark said her thoughts went out to his family, friends and colleagues who had suffered a tremendous loss.
"Rod Donald has remarkable energy and great zest for life. It is a cruel fate indeed which strikes down someone with so much to give in the prime of his life."
Mr Donald was first elected to Parliament in 1996 as a Green party member on the Alliance list.
An activist all his life Mr Donald was member of the Values party, took an active role in the anti-Springbok tour in 1981 and was a strong advocate for electoral reform.
Mr Donald's sudden death has shocked many because he was famed for his healthy lifestyle and robust good health.
His fellow co-leader Jeanette Fitzsimons said she was mourning his death.
"He is an incalculable loss to the Green Party, to the country and to me personally," Ms Fitzsimons said.
"I will miss his political wisdom, his generosity of spirit, his good humour and his sense of fun more than I can say."
Ms Fitzsimons said besides his key work in introducing MMP, he had worked tirelessly at community development, green issues and with Trade Aid.
"In Parliament he has fought for justice, for exploited Asian workers in sweatshops through the Greens' fair trade campaign and for the people of Zimbabwe in his efforts to highlight human rights abuses in that country."
Mr Donald had been deeply disappointed to not be part of the Government formed after the September election, however the Greens said they were determined to carry on the work he had begun.
Mr Donald had been given the job of Government spokesman on a "Buy Kiwi Made" campaign despite his party refusing to back Miss Clark's government.
National leader Don Brash said Parliament would not be the same without Mr Donald.
"Despite disagreeing on some policies, I admired Rod as a hugely principled, honest and capable man with a passion and a drive to represent his beliefs and speak his mind," Dr Brash told NZPA through a spokesman.
"Parliament will not be the same place without him."
Dr Brash offered his condolences to Mr Donald's family, friends and colleagues.
United Future leader Peter Dunne said he was devastated for Mr Donald's family.
The two leaders had their differences but Mr Dunne said he enjoyed working with Mr Donald when they were on the same side.
He admired his dedication, thoroughness and his "unfailingly cheery approach".
"I'm just shocked by that news," Mr Dunne told NZPA.
"It's an awful reminder to all of us that there's more to life than politics."
Progressive Leader Jim Anderton was Mr Donald's party leader as head of the Alliance and the pair fell out politically when the Greens left to go it alone.
"He came into the political arena via the Alliance, so I had a lot to do with Rod. He devoted his adult life to the cause of environmental issues and latterly the Green Party," Mr Anderton said.
"It will be a very big loss to the Greens and he made a significant contribution to New Zealand politics. It is such a sad loss at such a relatively young age."
Mr Anderton said despite their difference he always respected Mr Donald.
"Rod was a very dedicated person. He worked hard at everything he took on and had a passion with it. I did not always agree with Rod... but I never doubted his sincerity and commitment."
New Zealand First Leader Winston Peters said Mr Donald's untimely death was an enormous shock.
"Wherever one sits on the political divide, it can't be denied that Rod Donald was dedicated to his party's cause and their issues and had been a high profile and effective parliamentarian," Mr Peters said.
"Our condolences go to his wife, family, friends and supporters."
ACT leader Rodney Hide said his death was a big shock
"Rod Donald was a tireless campaigner for the Green Party, and he will be a big loss to the Greens."
Former Green MP Nandor Tanzcos, who lost his seat at the last election, is the next on the party list and would be entitled to succeed Mr Donald if he so chose.
Mr Tanzcos had just heard the news when NZPA called and said he was too distressed to comment or discuss the future.