UPDATED 4.20PM It's official - Helen Clark will be making a run at the UN Secretary General's job.

John Key said she'll have the full backing of the Government, saying there are major global challenges facing the world today and the UN needs a proven leader who can be pragmatic and effective.

"Coming from New Zealand, Helen Clark is well placed to bridge divisions and get results," he said.

LISTEN ABOVE: Helen Clark speaks to Newstalk ZB's Chris Lynch

Speaking to reporters at the Beehive, Mr Key said Helen Clark was "the best person for the job".

Having served as New Zealand's Prime Minister for nine years and one of the top UN roles for seven years, she had the right mix of skills and experience, Mr Key said.

The New Zealand Government is pleased to nominate Helen Clark @Helen4SG for the position of the UN Secretary-General.

— John Key (@johnkeypm) April 4, 2016

READ JOHN KEY'S NOMINATION LETTER TO THE UN

Helen Clark said in a statement: "To receive the full backing of the New Zealand Government is a great honour.

"New Zealand has a proud history of supporting the United Nations from its very beginning."

"We, New Zealanders, have developed our own way of getting along with one another and getting things done.

"The tradition of being tolerant, pragmatic, and fair is a central part of who we are, and I believe I would bring these attributes to the position of Secretary-General."

Today #NZ Prime Minister @johnkeypm announced Govt nomination of me as UNSG. Looking 4ward 2 the campaign. Follow me on @Helen4SG #Helen4SG

— Helen Clark (@HelenClarkUNDP) April 4, 2016

Ms Clark later gave a press conference at New Zealand's offices at the UN, which included a ceremony with a conch shell and waiata.

Addressing international media, Clark said she can offer the style of leadership that is needed to help the United Nations move ahead at a challenging time.

"We need a UN which is up to the task of tackling the major challenges facing the world today" and she believed she was the leader to do that.

She said she intended to run an "accessible" campaign and would set out her priorities in the course of the campaign.

Clark told Chris Lynch earlier this morning that she's been working on making the UN less secretive.

"In the most recent international aid transparency rating, UNDP was ranked as the most transparent and accountable development agency in the world" which is incredible achievement," she said.

John Key said although Helen Clark clearly had the most experience out of the Secretary-General candidates, he said there was "a lot of horse trading" in choosing the next UN leader.

Bulgaria's former foreign minister and current UNESCO director-general Irina Bokova has also put her hand up, and is the current front-runner.

Others include the European Commissioner for Budget and Human Resources Kristalina Georgieva, as well as a former Serbian Foreign Minister, a former Macedonian Minister of Foreign Affairs, a Slovakian who's currently the Head of the UN Assistance Mission in Afghanistan, Montenegro's former Prime Minister, Croatia's Deputy Prime Minister, the former president of Slovenia, and former Prime Minister of Portugal.

Other names bandied about appear unlikely because of their partisan politics or prickly personalities - they include Israel's Tzipi Livni, German Chancellor Angela Merkel and Australia's Kevin Rudd.

John Key said he lobbied global leaders during a visit to Washington last week, and directly raised her candidacy with US President Barack Obama.

He said he did not get any "push back" from the US when he discussed her bid.

The Prime Minister said it was difficult to choose a frontrunner, because the process was highly politicised and complicated. The public should have a "degree of confidence and a degree of realism" about Helen Clark's chances, Mr Key said.

The New Zealand Government would spend "hundreds of thousands" in supporting her bid.

Opposition parties are joining the Government in backing of Ms Clark. Labour Leader Andrew Little who said he feels proud she's decided to stand for this leading international role.

Little said Clark would make a great successor to Ban Ki Moon when he steps down at the end of the year.

"She would make a fantastic secretary-general," he said. "I think it's great that a woman from New Zealand, the country that first gave women the vote, should be involved in breaking the glass ceiling at the UN."

New Zealand First leader Winston Peters said his party's pleased the Government has followed the long tradition of supporting prominent New Zealanders on the international stage.

"Anyone in her position obviously, we would seriously support in our national interest in terms of our profile, make no bones about it".

And Green Party Co-leader Metiria Turei is also endorsing Helen Clark, saying it's time for a woman to lead the UN and Ms Clark's got the experience and integrity to do the job.

Former Prime Ministers are among those rallying behind Helen Clark.

Mr Key said he expects former Prime Ministers and prominent ex-politicians to take on roles as her informal envoys.

Former Prime Minister Jim Bolger said the Government made the right move in nominating Clark.

And former Labour Prime Minister Sir Geoffrey Palmer said he will do whatever he can to help Ms Clark's cause with his international contacts.

Dame Jenny Shipley is yet to comment on the nomination however, and former Labour Prime Minister and WTO director-general Mike Moore has refused to comment.

Ms Clark will have more than 40,000 staff under her if she is appointed the next Secretary General of the UN.

She will also earn a tax-free base salary of about $340,000 New Zealand dollars and receive free accommodation in New York.

It is the most difficult job in the world according to Trygve Lie, the man who first held the role.

By comparison, in her current UNDP role Ms Clark oversees 7,500 staff for a base salary of $280,000 a year.