Auckland Mayor Phil Goff's chief of staff, Fran Mold, has resigned to take up a job with the United Nations in war-torn South Sudan.

Mold will work alongside former Labour Party leader David Shearer, who has the tough job of leading the United Nations' mission in South Sudan.

Mold said she had wanted to work overseas in a humanitarian environment and when the opportunity came up "I grabbed it with both hands".

"I have thoroughly enjoyed the [mayoral] job. It has been challenging and intense and I am proud of what we have done in the first six months," she said.

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The former New Zealand Herald journalist, who worked in Parliament as Shearer's chief of staff, has just completed a Masters of Philosophy in politics with a thesis on communications used by Isis and Hamas to achieve political objectives.

Her new job is director of public engagement for the UN in South Sudan.

Mold leaves before Easter. Goff's chief press secretary, Nirupa George, will be acting chief of staff until June when a new, as-yet unnamed person will fill the role.