Labour MP David Shearer is tipped to resign to lead the United Nations' mission in war-town South Sudan.
A recommendation has been made to the UN Security Council in New York by Secretary General Ban Ki-Moon. It's expected to be approved this week, with Mr Shearer's previous experience at the UN meaning he's unlikely to face any opposition.
Foreign Affairs Minister Murray McCully says the Government is backing Mr Shearer's appointment.
"It is a huge deal," he told the New Zealand Herald. "[Security Council members have] a couple of days to raise any concerns, so it is not a done deal yet, but it is a big feather in his cap."
Mr Shearer spent nearly 20 years working for the UN before entering politics. There he worked in the Balkans, Rwanda, Somalia, Iraq, Albania and Serbia.
He spent nearly two years as leader of the Labour Party, replacing Phil Goff and later being rolled by David Cunliffe.
Mr Shearer's departure would trigger a by-election in Mt Albert, a strong Labour seat which was previously held by Helen Clark.
It neighbours Mt Roskill, which recently voted in a by-election that saw Labour's Michael Wood win in a landslide.
Labour leader Andrew Little says he's known for a while now Mr Shearer wanted to quit.
"He's been actively looking for alternative roles for some time. He decided some time ago that he wanted to continue to do the types of roles he was doing before he came into Parliament - he's been open and honest with me about that."
Mr Little says while disappointing for Labour - losing "the best Foreign Affairs Minister we are yet to have" - Mr Little is happy for Mr Shearer.
"This is him carrying through a passion in his life… I want him to be satisfied about what he's doing. I think he's just champing at the bit to get on with something he can see has immediate practical effects.
"It's always a bit dispiriting in Opposition. You get to talk about a lot of stuff, but you don't get to make decisions that make a difference."