But Power said it was difficult to find a candidate that could help achieve a "Venn diagram" between Russia, China, and the US at the time.
"That was the real challenge, I think, that we faced.
"Even having someone of Helen's calibre in the race, I think, created a race to the top in many respects."
Power said there was a push for a female Secretary-General, and while they did not achieve that, Guterres was focused on promoting women's equality in his work.
"Guterres was somebody who, like Helen Clark, had led a country . . . he was missing an important quality that many were looking for, which is a different gender."
She said even he had been able to act as the "Venn diagram" between the three nations.
"I don't know that Helen Clark would have had more luck."
Clark stepped down from her role as head of the UN's development programme after the loss.