“I fully intend to be contactable and available throughout the six-week period when needed.”
Ms Ardern said she discovered her pregnancy only six days before learning she would be prime minister, and it was “100% a surprise”.
“I am not the first woman to multi-task. I am not the first woman to work and have a baby – there are many women who have done this before,” she said.
Mr Gayford would be a “stay-at-home” dad, she added.
Two of New Zealand’s former prime ministers were among the first to offer congratulations.
Wishing @jacindaardern & @NZClarke all the best as they expect their 1st child in June: a super busy year coming up & much to look forward to. Every #woman should have the choice of combining family & career. https://t.co/Ma6B6OGXJe
Ms Ardern took over the Labour leadership in July. She later said that she had an hour’s notice that Andrew Little was about to stand down.
On her first day as opposition leader, Ms Ardern was controversially asked by a TV talk show host whether she had made “a choice between having babies and having a career”.
Ms Ardern replied that such a question was “totally unacceptable in 2017”.
“It is a woman’s decision about when they choose to have children and it should not predetermine whether or not they are given a job or have job opportunities,” she said.
Ms Ardern’s announcement is rare for world leaders.
In 1990, Benazir Bhutto gave birth to a daughter while serving as Pakistan’s prime minister, in what was reported to be a first for an elected world leader.