Former Labour Prime Minister Helen Clark could be in the running for the UN's top job.

Nominations for the United Nations top job have opened and John Key says his government will support Helen Clark if she applies.

The former Labour Prime Minister has never commented on whether she wants the position of Secretary-General but women have been encouraged to apply.

Clark holds the post of administrator of the United Nations Development Programme, making her the third-highest ranked person at the UN.

LAWRENCE SMITH/FAIRFAX NZ Applications for the job of UN Secretary-General have opened.

UN General Assembly president Mogens Lykketoft said in a statement at the UN's New York headquarters that he encouraged member states "to consider presenting women, as well as men, as candidates for the position of Secretary-General".

Lykketoft highlighted a joint letter with the president of the Security Council about the process of soliciting candidates leading to the selection and appointment of the next UN head.

The letter said the position was one of great importance that required "the highest standards of efficiency, competence and integrity, and a firm commitment to the purposes and principles of the Charter of the United Nations".

"Convinced of the need to guarantee equal opportunities for women and men in gaining access to senior decision-making positions, Member States are encouraged to consider presenting women, as well as men, as candidates for the position of Secretary-General."

There was a "strong wish from many" for a woman to take on the top job, Lykketoft told reporters.

A spokesman for John Key told Radio New Zealand the government would offer Clark its full support if she chose to put herself forward for the role, but she had not yet done so.

Labour's foreign affairs spokesperson David Shearer said he thought she would.

"She will definitely have backing of New Zealand. She would have a good chance so she will, I think, have a crack at it.

"The word on the street is that she's been for some time talking about it and testing out the waters with various people," Shearer told Radio New Zealand.

This would be the first time all 193 member states would be involved in the selection of the UN Secretary-General.

Two people have already been nominated, former president of the general assembly Srgjan Kerim of Macedonia and Croatian foreign minister Vesna Pusic, who is a woman.

The next Secretary-General will assume the role in January 2017 and will serve a five year term, which can be renewed by member states for an additional five years.

The Secretary-General also acts as the de facto spokesperson and leader of the United Nations.

They will take over from current Secretary-General Ban Ki-moon of South Korea, who took office in January 2007 and has served two five-year terms.