Prime Minister Malcolm Turnbull has confirmed he will not be nominating Kevin Rudd to be the next secretary-general of the United Nations.

Key points: PM will not divulge reasons why Rudd is not suited to the position

PM will not divulge reasons why Rudd is not suited to the position Labor says decision is a sign of weakness from PM

Labor says decision is a sign of weakness from PM 12 people are nominated for the UN position

Speaking in Sydney today, Mr Turnbull said he had decided the former prime minister was not suitable for the role.

He said the Federal Government would not be nominating anyone for the role.

"When the Australian Government nominates a person for a job, particularly an international job like this, the threshold question is, 'do we believe the person, the nominee, the would-be nominee is well suited for that position?'" he asked.

"My judgement is that Mr Rudd is not, and I've explained to him the reasons why."

Kevin Rudd's hopes of being UN secretary-general have been dashed ( Getty Images: ChinaFotoPress )

Mr Turnbull refused to be drawn on the reasons why he thought Mr Rudd was unsuitable for the role, or on whether his decision was about satisfying certain members of his party.

He said this decision was "far from the most important issue confronting the Government".

Foreign Minister Julie Bishop had pushed Mr Rudd's case during yesterday's Cabinet meeting, with support from Attorney-General George Brandis.

After the meeting the PM said he alone would make a decision on the issue, after Cabinet failed to reach a clear decision.

Nomination would 'reflect well' on Australia: Rudd

Mr Rudd said he flew to Sydney to meet with the Prime Minister, but "Mr Turnbull telephoned Mr Rudd, indicating there was no opportunity for a meeting."

In a statement on his Facebook page, Mr Rudd said his nomination would have "reflected well on what our nation can offer to the world".

"A nomination by the Government would not have granted Mr Rudd a position," it read.

"It would simply have enabled him to stand alongside the 12 other candidates from across the world, and compete on his merits. That is now not to be.

"It is a pity the Government has not seen fit to support him, as the Hawke government supported Malcolm Fraser for the post of secretary-general of the Commonwealth, or the Howard government supported Gareth Evans to be head of UNESCO."

The statement also thanked Ms Bishop for her support, adding: "Mr Rudd remains a fervent defender and advocate of the UN in these difficult times."

Decision shows Turnbull is weak, Labor says

Acting Opposition Leader Tanya Plibersek slammed the decision, which she said undermined the authority of both Mr Turnbull and Ms Bishop.

Sorry, this video has expired Tanya Plibersek slams decision not to back Kevin Rudd

Speaking in Townsville, Ms Plibersek said "the right-wing extremists in the Liberal Party have got the Prime Minister on a short leash".

"He was not able to insist that what any right-thinking Australian would think, that an Australian should be backed for an international job like this," she said.

"He was not able to get that through his Cabinet. It shows the weakness of the Prime Minister, it shows the weakness of the Foreign Minister."

Labor frontbencher Anthony Albanese tweeted: "Turnbull. Pathetic".

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Mr Rudd had previously received vocal support from former Liberal leader Brendan Nelson, though South Australian Liberal senator Cory Bernardi had urged his colleagues not to endorse a "dysfunctional", "vengeful", "unstable", "megalomaniac" like Mr Rudd.

Senator Bernardi released a statement after Mr Turnbull made his announcement, congratulating the Prime Minister on his decision and saying: "Our participation in international institutions is more important than an individual's ambition".

Australia should back former NZ PM: Beazley

There are already 12 nominees for the position, including former New Zealand prime minister Helen Clark.

Ms Clark has received support from the New Zealand Government, whose Prime Minister John Key has lobbied on her behalf and provided funding for her campaign.

Former Croatian minister of foreign and European affairs Vasna Pusic, UNESCO director-general Irina Bokova, and Argentine Foreign Affairs Minister and former chief of staff to UN Secretary-General Ban Ki-moon, Susana Malcorra, are also among the nominees.

Former Labor leader and ambassador to the US, Kim Beazley, told ABC's PM program there was a "strong appetite" for female leadership.

Mr Beazley said Mr Turnbull made the right call.

"I think we should be supporting a woman for the job at the United Nations," he said.

"There are several well-qualified women in the race.

"We should be putting our shoulders to the wheel behind one of them as the next secretary-general … my advice to the Government, for what it's worth, would be to get behind Helen Clark."