Labour MP David Shearer is set to leave Parliament, which could prompt a byelection in his Auckland electorate.
Mr Shearer has been the MP for Mt Albert since 2009 and is the party's foreign affairs spokesperson.
He has been put forward for a job leading the United Nations' mission in South Sudan.
United Nations Secretary-General Ban Ki-moon said he had notified the Security Council of his decision to Mr Shearer as the head of the peacekeeping mission.
Mr Ban said Mr Shearer would bring extensive political and humanitarian experience to the role.
If there are no objections, the Security Council will approve the appointment on Friday.
Mr Shearer would succeed Ellen Loj of Denmark in the position.
Foreign Affairs Minister Murray McCully said Mr Shearer would be the UN secretary general's representative on the ground in South Sudan, leading a major operation in a very difficult place.
"Its the toughest peacekeeping assignment on the planet."
His name was the only one put forward and if appointed he would oversee a $1.5 billion operation and 18,000 people. Mr McCully said the proposed appointment had "the full support of the New Zealand government."
Labour Party leader Andrew Little said the party was prepared for a byelection in Mt Albert - or the possiblity of an early general election.
The planned departures of Mr Shearer, National's Maurice Williamson and Labour's David Cunliffe, along with with an unelected Prime Minister taking office, may be grounds for an early election, he said.
"These may well be the conditions to go early. If so, we're ready to go." - Andrew Little
The Security Council has yet to confirm the appointment, which would likely have to be taken up by the end of January.
However, if Mr Shearer stays on as Mt Albert's MP until June, a byelection would be avoided as the general election would be only months away. If he leaves before then residents would need to vote.
A byelection in Mt Albert would be the second inner city Auckland vote in a short period of time. Last weekend Mt Roskill residents voted in a byelection, sparked by Labour MP Phil Goff stepping down to become Auckland mayor. Labour's Michael Wood won the seat by a landslide.
Mr Shearer became leader of the Labour Party in 2011, replacing Phil Goff. He held the position for a little under two years, standing down mid-term in August 2013, saying he no longer had the confidence of many caucus colleagues.
Mr Shearer said this morning he would be making no comment until the Security Council made its decision on the appointment.
He has previously worked for the United Nations for almost 20 years. He ran humanitarian operations for the UN and Save the Children in the Middle East and Africa.