Labour MP Charles Chauvel is resigning from Parliament to take up a job with the United Nations in New York.
Mr Chauvel, a List MP, said he had resigned effective from March 11.
He will work for the UN Development Programme organisation former Prime Minister Helen Clark leads - as an advisor on Parliamentary Development and Democratic Governance.
Mr Chauvel said Ms Clark was not involved in his selection - "I don't think she even knows I have the job."
He said he had been a member of the UN Global Commission on HIV and the Law for the past two years and had found that work challenging.
"The new full-time role with the UN presents a similar opportunity to make a difference and I look forward to the challenges that it will present."
Former MP Carol Beaumont is next on Labour's list - followed by Kelvin Davis.
Labour leader David Shearer said Mr Chauvel had played an important role within Labour and would be missed.
"We wish Charles well with his new role. He will do New Zealand proud in this international assignment," said David Shearer.
Mr Chauvel was the party's shadow Attorney-General and spokesperson for Justice and Arts, Culture and Heritage.
A former lawyer, Mr Chauvel entered Parliament in August 2006 as a List MP, replacing Jim Sutton who retired.
In the former Labour Government, he was chair of the Privileges Committee, chair of the Finance and Expenditure Committee, and Private Secretary to the Attorney General.
The changeover will coincide with Mr Shearer's planned reshuffle, but Mr Chauvel said that had not played a part in his decision. There had been speculation that Mr Chauvel could be demoted as a punishment for his support of David Cunliffe.
Mr Chauvel said he had only made the final decision to take up the job at the UN this morning and had told Mr Shearer immediately. He said Mr Shearer had not given him any indication about what his fate might be in the reshuffle prior to that - and he would have taken the UN job even if he had been told he was due to be promoted to the front bench.
He said the job had first been raised as a possibility last year and it had been under discussion since then. He will work with developing countries on improving their democratic governance structures, predominantly in Africa and Asia.
Carol Beaumont said she was looking forward to returning to Parliament and could start as soon as required. She had no advance notice of Mr Chauvel's intention to resign.
Ms Beaumont was a List MP from 2008 to 2011, but Labour's low election result meant she did not make it back in in 2011.
She was the candidate for the party in Maungakiekie, which Labour lost to National's Sam Lotu-Iiga in 2008.
Since then, Ms Beaumont said she had worked as a consultant on short-term projects, including assisting the Maritime Union with the ports dispute, observing elections in Jordan and acting as a Convenor for the National Council of Women.