Green Party co-leader MP Jeanette Fitzsimons confirmed on Monday she will stand down from the position in June.
After 14 years as co-leader, Ms Fitzsimons will step down in June, but will remain in Parliament as a backbench MP until the next general election in 2011.
Ms Fitzsimons had been expected to retire before the 2008 election but continued after the sudden death of co-leader Rod Donald in 2005.
Last year, she said she could stand down as co-leader this year to give her successor time to settle in before the next election.
The Green's constitution requires the party's co-leaders be a man and a woman.
Her successor will be elected at the party's annual conference in June.
MPs Sue Bradford and Meteria Turia have announced their intention to nominate themselves as candidates. Other nominations may come from party members.
Tributes for Fitzsimons
Prime Minister John Key paid tribute to Ms Fitzsimons on Monday.
Mr Key says though National and the Greens have not always agreed on policy positions, he respects the way Ms Fitzsimons has carried out her role with integrity and belief.
Labour leader Phil Goff says Ms Fitzsimons is respected across the House for the dignity and integrity with which she has conducted herself and the affairs of her party.
Former Greens MP Nandor Tanzcos says while the party will continue to go from strength to strength, there is no doubt that Ms Fitzsimons' unique skills will be missed.