Labour MP Charles Chauvel says he will resign his seat on March 11.
The one-time party high flyer says he has accepted a position with the United Nations in New York (workplace of former Prime Minister Helen Clark), advising on parliamentary development and democratic governance.
Mr Chauvel was pegged by media as a member of "Camp Cunlifee" during the recent Labour leadership spat.
As the incumbent David Shearer consolidated his position in February, easily winning a leadership vote, Mr Chauvel found himself on the outer.
A former Minter Ellison partner, Mr Chauvel entered parliament as a list MP in 2006 as a list MP.
He has served as Labour's justice spokesman since February 2011 and as the shadow atorney-general and Labour's arts, culture and heritage spokesman since December 2011.
For the past two years, the openly gay Mr Chauvel has also been a member of the United Nations' Global Commission on HIV and the Law, a part-time position.
Former MP Carol Beaumont is next on Labour's list.