Wellington: New Zealand Prime Minister Jacinda Ardern has announced she is expecting her first baby, and is set to become the country's first leader to give birth while in office.

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 yesterday 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. 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.