SYDNEY, Australia — New Zealand’s prime minister, John Key, surprised the country on Monday by announcing he would resign next week, citing a desire to spend more time with his family.

Mr. Key, 55, has been in office since November 2008 and led the conservative National Party to election victories in 2011 and 2014. The next election is scheduled for next year, and Mr. Key had been seen as the likely favorite.

Though many people were asking why, answers were elusive.

“I think he decided he was simply done with politics particularly when this year his friend David Cameron in the U.K. decided to step down as prime minister and another friend of his, Barack Obama, will next year leave office,” said Stephen Levine, a professor of political science at Victoria University of Wellington.

Mr. Key, a former Merrill Lynch executive, said Monday that he had never wanted to be a career politician and did not know what he would do next.