Green Party co-leader Jeanette Fitzsimons is expected to announce she will stand down on Monday morning.
Ms Fitzsimons has been co-leader of the Greens since 1995.
She entered Parliament with late co-leader Rod Donald in 1996 under the Alliance Party banner before returning after the 1999 election as one of seven Green MPs.
She was expected to retire before last year's election but agreed to stay on after the sudden death of Mr 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 Party's constitution requires the party's co-leaders be a male and female.
Her successor would be elected at the Green's annual conference in June.
