UPDATED 8.02am The country's set for another by-election and this one's coming as a shock.

LISTEN ABOVE: Murray McCully spoke to Mike Hosking

David Shearer's set to quit Mt Albert, to take up a job with the United Nations, leading its mission in South Sudan.

A recommendation for the Labour MP's appointment has been put before the United Nations Security Council by Secretary-General Ban Ki Moon.

That's set to be approved this week.

All Security Council members have two days to object but Mr Shearer's previous experience means he's likely to get the tick.

Mr Shearer will work alongside the commander of 18,000 peace-keepers, with a budget of about $1 billion.

He released a statement this morning saying the Security Council would be making a decision on the post.

“My name has been proposed to the United Nations Secretary General to be his Special Representative in South Sudan.

“The matter is currently before the Security Council for its decision.

“Until the Security Council completes its consideration of the appointment I will not be commenting further.”

Foreign Minister Murray McCully told Mike Hosking the nomination hasn't come from New Zealand.

"We obviously strongly support David Shearer in this process but his personal experience and capabilities have led the UN to offer him this position.

"It's already been approved by Ban Ki Moon, and his successor Antonio Guterres, so it's not done and dusted but it would be very unusual for it to get derailed at this point," Mr McCully told Mike Hosking.

He describes it as a huge deal, and a feather in Mr Shearer's cap.

Labour leader Andrew Little said the position, the Special Representative and head of UNMISS (United Nations Mission in South Sudan), is regarded as “the most challenging of peacekeeping operations currently”.

UNMISS is the UN’s third largest peacekeeping mission with 17,000 uniformed and almost 2000 civilian personnel. It is charged with building peace and stability in what is the youngest country in the world.

Mr Little said he first heard from Mr Shearer two weeks ago that he had been offered the role at the UN and that he wanted to accept it.

Mr Little said Shearer had kept him “fully informed about this opportunity and we are very excited for him.”

“Should he be confirmed, David will be the only New Zealander in charge of a UN peacekeeping mission. This is a very exciting opportunity for him to make a difference and help bring peace to a country of nine million people torn apart by civil war.

“We fully understand his reasons. This is a once-in-a-lifetime opportunity to take up a senior position at the United Nations which reports directly to the Secretary General.” 

But the political bombshell means a by-election is now looming early next year.

Labour leader Andrew Little said his preference is for a by-election to be avoided, and to have an early general election instead.

“We have anticipated that there could be an earlier election and more so given the change of leadership in the National Party.

“So I think I’d probably rather avoid a by-election and just get straight into an earlier election.”

Mr Little said there had been no thought over that time about who should run in the Mt Albert seat for Labour in place of Mr Shearer, but people would be queuing up.

National lost the recently held by-election in the neighbouring Mt Roskill electorate by a huge margin.

Mr Shearer won Mt Albert in 2014 with a majority of 10,656.

He was elected to Parliament in a by-election in 2009, replacing Helen Clark.