They might not have the same ring as

Thelma and Louise

or Simon and Garfunkel, but Helen and Heather have had a more enduring and harmonious partnership than either of them.

But the departure of Helen Clark from the United Nations Development Programme in April could see the end of a 30-year successful political partnership with Heather Simpson.

Simpson was chief of staff for the nine years Clark was Prime Minister. For the past eight, she has been a special advisor to Clark at the UN, where the former PM held the number 3 position.

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Simpson helped Clark reform the UNDP which included cutting staff and providing greater transparency about spending and work in developing countries with its $US5 billion ($6.88 billion) annual budget.

She was also at UNDP during the crucial setting by the UN of the 2030 Agenda for Sustainable Development, the next set of goals to elimate poverty.

Simpson is in that rare club of movers and shakers who has as much acuity around policy and strategic direction as she does around politics.

She elected to stay with the UNDP during Clark's unsuccessful bid to become UN Secretary General last year.

Clark will leave after two four-year terms in the job but has given no indication if she has a new job lined up, whether she is in the job market or just wants some time off with family.

She has been running a long-distance marriage with her academic husband, Peter Davis, who lives in Auckland. Her elderly father, George Clark, whom she phones every day, is based in Waihi Beach.

Helen Clark turns 67 next month. Simpson is 63.

Their working relationship goes back to 1987 and the Fourth Labour Government.

When Clark held the Housing, Conservation and Health portfolios and Simpson was a policy adviser to her.

Simpson then became director of policy and research for the parliamentary Labour Party before her elevation to chief of staff when she became Clark's "enforcer" in the various political crises and scandals that arose as well as policy overseer.

She earned the nickname H2, as the second-most powerful woman in New Zealand, next to Helen Clark, H1.

A former economics lecturer at Otago University, she also twice stood for Parliament for Labour, in Awarua in 1987 and in Heretaunga in 1993.

Simpson was known to be initially reluctant to head to New York to work at UNDP.

But according to friends, Simpson and her civil union partner, Sue Veart, feel very settled in New York and love the cultural life.

That could make a shift with Clark to a new job difficult if it were in new city.

Neither Clark nor Simpson responded to a requests for an interview.