A senior former Australian diplomat adds his support to Kevin Rudd's bid to become the next secretary-general of the United Nations.

Key points: Cabinet's endorsement "essential" for the credibility of Rudd's bid

Cabinet's endorsement "essential" for the credibility of Rudd's bid 13 candidates in the running for top UN job

13 candidates in the running for top UN job Mr Rudd's campaign not acknowledged by Coalition until Ms Bishop briefed the party room

Richard Woolcott, who served as Australia's top diplomat in the UN for six years, said the Federal Cabinet must support Mr Rudd by formally nominating him.

"If an Australian decides to stand, the Australian Government should support that Australian," Mr Woolcott said.

There are 13 candidates in the running for the top UN job, including former New Zealand prime minister Helen Clark.

It was only when the Foreign Minister Julie Bishop briefed the Coalition party room on Monday that Mr Rudd's campaign for the UN secretary-general position was formally acknowledged.

Diplomats, including Mr Woolcott, say Cabinet's endorsement is essential for the credibility of the two-time former prime minister's bid.

Mr Woolcott said Mr Rudd's candidacy would effectively be killed off if he did not receive Cabinet's nomination.

"I think that one of the sort of loose rules applied is that you have to have the support of your own government," he said.

Needs support from the 'big five' of the Security Council

As well as his time at the UN, Mr Woolcott has held positions as Australia's ambassador to Indonesia and the Philippines, as well as High Commissioner to Malaysia, Ghana and Singapore.

He said the 13 declared candidates needed the support of the many, but most importantly, the few, within the UN.

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"There's a General Assembly committee which interviews the candidates and presumably makes some recommendations," he said.

"But in the end, you can't be elected if one of the 'big five' veto you, so that means that you really can't get elected if anyone of the permanent five members of the Security Council veto you."

Mr Woolcott said Mr Rudd had been building a campaign strategy around China, France, the UK, the US and Russia too.

"He is a well respected and well known by Xi Jinping, the Chinese head of state," Mr Woolcott said.

"I think the same would probably apply to Britain, and France of course, and the United States.

"His problem could be Russia because of the way the relationship with Russia deteriorated when Tony Abbott was prime minister, when he was talking about shirt-fronting Putin."

Perhaps for that reason Mr Rudd has travelled to Russia at least four times, and the visits have included face time with prime minister Dmitry Medvedev, and as much time as possible on TV talking reassuringly about world affairs.

Obstacles remain from Bulgaria and New Zealand

Big obstacles remain though for Mr Rudd, with many seeing it as Eastern Europe's turn to hold the secretary-generalship.

The UNESCO director-general, Bulgarian Irina Bokova, stands as the region's frontrunner.

But Mr Woolcott said he thought Ms Bokova had one large problem of her own.

"She could well have problems with the United States, could possibly be vetoed by the United States," he said.

The same fate, Mr Woolcott said, could face the big-name candidate, Ms Clarke.

"So she could be opposed by the United States because of her opposition when she was prime minster of New Zealand to the nuclear ships in New Zealand."