Former New Zealand Prime Minister Helen Clark is hoping to be the UN's next Secretary General.

Helen Clark is trailing the top contenders in a vote to decide the next United Nations boss in a shock result that some are reading as a deliberate attempt to nobble her prospects.

Clark has emerged in the "middle of the pack" of 12 contenders for UN Secretary General after a straw poll of the UN Security Council members.

The secret ballot on Friday morning was a bid to whittle down the candidates, as Secretary-General Ban Ki-moon is due to step down at the end of 2016 after two five-year terms.

MIKE SEGAR/REUTERS Former Slovenian President Danilo Turk, one of the frontrunners after the poll, is seen here with former United Nations climate chief Christiana Figueres (left) and ex NZ Prime Minister Helen Clark, right.

The two leading contenders are former Portuguese prime minister Antonio Gueterres and former Slovenian president Danilo Turk.

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There are already claims being made about the "old boys network" holding sway after a clear mood among the wider United Nations for a woman Secretary General.

SAM SACHDEVA/Stuff.co.nz Prime Minister John Key says Helen Clark shouldn't be overly worried if Kevin Rudd joins the race to become the UN's next secretary-general.

Clark is still in the running after coming in between fourth and sixth, but Eastern European woman candidate Irina Bokova is ahead of her.

Clark has campaigned strongly but in a surprise move received more votes discouraging her candidacy than expected, which some say was a clear attempt to nobble her candidacy.

But it seems her greatest obstacle may be overcoming a mood for an Eastern European candidate.

PROCESS OF FINDING A NEW CHIEF

The 193-member UN General Assembly has this year sought to lift a veil of secrecy that has surrounded the election of the UN chief for the past 70 years by requiring public nominations and holding campaign-style town hall events with each candidate.

However, it is the 15-member Security Council which will choose a candidate to recommend to the General Assembly for election later this year.

The council will continue to hold closed-door informal secret ballots until they reach consensus.

On Friday, council members were given a ballot for each candidate with the options of encourage, discourage and no opinion.

The nominating states will be told of the results for their candidate, but overall results will not be made public.

The search for a successor to Ban, a former South Korean foreign minister, has sparked a push by more than a quarter of the 193 UN states for the world body's first female leader, and half of the current candidates are women.

The council hopes to agree on a candidate by October, diplomats say.

Former Australian prime minister Kevin Rudd has asked the Turnbull government to back his nomination for the post, but no decision has so far been taken.

The female candidates so far are: UN cultural organisation UNESCO Director-General Irina Bokova of Bulgaria; former Croatian Foreign Minister Vesna Pusic; Moldova's former Foreign Minister Natalia Gherman; former New Zealand Prime Minister Helen Clark, who heads the UN Development Programme; Argentinian Foreign Minister Susana Malcorra; and former UN climate chief Christiana Figueres of Costa Rica.

Also in the race are Montenegro Foreign Minister Igor Luksic; former Slovenian President Danilo Turk; former UN High Commissioner for Refugees Antonio Guterres, who is also a former Portuguese Prime Minister; former Serbian Foreign Minister Vuk Jeremic; former Macedonian Foreign Minister Srgjan Kerim; and Slovak Foreign Minister Miroslav Lajcak.