While the Federal Government appears undecided whether former prime minister Kevin Rudd should be the UN secretary-general, in New Zealand there has only been praise for fellow candidate, former prime minister Helen Clark.

Malcolm Turnbull is yet to endorse Mr Rudd for the role.

Cabinet is expected to vote next week, but the Prime Minister has declined many opportunities to sing Mr Rudd's praises.

The two led the opposing sides of the Australian political aisle for more than a year — half the time Ms Clark faced then-opposition leader John Key.

In the past Mr Turnbull described Mr Rudd as "self-serving" and said "Rudd likes to style himself as a philosopher-king issuing edicts from on high about how the world should be governed".

Mr Key, now New Zealand's Prime Minister, was once no kinder to Ms Clark.

He accused her of being boring, a relic and lacking solutions as he prepared to take her on in the 2008 election.

Ms Clark dished it back, describing Mr Key as a "flip flop".

Key throws support behind Clarke

However, much has changed. Ms Clark's bid to be the next United Nations secretary-general was announced by Mr Key in the Beehive Theatrette, a venue reserved for official announcements backed by the full force of government.

And Mr Key does not usually hold press conferences in the morning, but this one was held at 7:30am to coordinate with Ms Clark.

Mr Key recently said if the election of UN secretary-general "was a straight horse race on who has got the most ability, I think Helen Clark would win".

The UN Security Council will vote on the candidates and deliver its recommendation to the General Assembly, so Ms Clark's support from the five permanent members, who have the power of veto, and the 10 non-permanent members is crucial.

The council conducts a series of straw polls via colour-coded papers until they arrive at a consensus.

Ms Clark has a slight advantage. She is the only candidate from a country that is currently sitting on the Security Council.

Mr Key has made a point of lobbying on Ms Clark's behalf when speaking with world leaders, including Chinese President Xi Jinping, US President Barack Obama, former UK prime minister David Cameron and French President Francois Hollande — the leaders of every permanent member of the Security Council except Russian President Vladimir Putin.

Ms Clark's travel expenses for her campaign are covered by the New Zealand Government, and several staff members have been seconded from the Ministry of Foreign Affairs and Trade to help.

She has travelled to Venezuela, Ukraine, Germany, Russia, China, Malaysia, Spain, Senegal, Uruguay, France, the USA, Tonga, Japan, Indonesia, Denmark, the UK, Norway, Kenya, Austria, Egypt and Dubai since her candidacy was announced in early April.

Her campaign has a strong social media presence and she has tweeted pictures from all of these locations.

The exact bill for the campaign has not been made public, but Mr Key expects it to cost hundreds of thousands of dollars.