The 37-year-old, who was sworn in last October, made global headlines when she slapped down pre-election questions over whether she intended to start a family, insisting pregnancy had no bearing on a woman's career opportunities.
Ardern was all smiles Friday as she appeared with partner Clarke Gayford at their home to announce the "unexpected but exciting" news of their first baby.
"Clark and I are really exited to share...that in June we are looking forward to welcoming our first child," she told reporters.
"We still have to get used to saying that out loud because we've been keeping that to ourselves for quite a long time."
The charismatic leader enjoyed a rapid rise to the top ranks of politics, winning office last year just months after taking the helm of the centre-left Labour Party.
"We thought 2017 was a big year!" she tweeted.
"This year we'll join the many parents who wear two hats. I'll be PM and a mum while Clarke will be 'first man of fishing' and stay at home dad."
Ardern said she would take six weeks off after the birth of her child, with maverick Deputy Prime Minister Winston Peters taking the reins of office.
She said she aimed to be "contactable and available" during the period and would resume all leadership duties when it was over.
Ardern, who did not reveal whether she was expecting a boy or a girl, said she and Gayford previously had doubts they could conceive.
"Clarke and I have always been clear we wanted to be parents but had been told we would need help for that to happen," she said.
"That's made this news a fantastic surprise."
She tweeted a picture of two large fishing hooks, one with a smaller hook inside it, in reference to Gayford's career as a television fishing show presenter.
- Sexism row -
Ardern's plans for a family sparked a sexism row during the election when a television host quizzed her on the issue, saying voters had a right to know before they cast their ballots.
She rejected the line of questioning as "unacceptable", saying pregnancy and child rearing should not hinder women's opportunities in the workplace.
"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.
While several male prime ministers have become parents in office -- including Britain's Tony Blair -- late Pakistani prime minister Benazir Bhutto is believed to have been the first head of government to have given birth during her term, when she had a baby in 1990.
Ardern said "there's no doubt times have changed" making it possible for her to juggle motherhood and a high-profile political career.
But she played down suggestions she was a trailblazer.
"I am not the first woman to multitask. I am not the first woman to work and have a baby," she said.
"I know these are special circumstances, but there are many women who would have done this well before I have.
"There are plenty of women who carved a path incrementally to make it possible for people to look upon my time in leadership and think 'yes I can do the job and be a mother'."
- 'More excited than election' -
Australian Prime Minister Malcolm Turnbull offered his congratulations on the "wonderful news." When we spoke this morning you sounded more excited than you did when you won the election!" he tweeted.
Ex-New Zealand prime minister Helen Clark, Ardern's mentor early in her career, said her former charge was in for "a super busy year".
"Every woman should have the choice of combining family & career," she tweeted.
Green Party co-leader James Shaw said Ardern's announcement was significant for many women.
"That a woman can be the prime minister of New Zealand and choose to have a family while in office says a lot about the kind of country we are and that we can be -- modern, progressive, inclusive, and equal," he said.
Gayford was non-committal when asked if the pregnancy meant he would now marry Ardern.
"Wow, I like the idea that we are doing everything in reverse," he told reporters.
"We bought a house together, then we are having a baby and will see..."//AFP