WELLINGTON (REUTERS, BLOOMBERG) - New Zealand Prime Minister Jacinda Ardern said on Friday (Jan 19) that she was pregnant with her first child, which was due in June.

Ardern, 37, and her long-time partner, TV fishing show presenter Clarke Gayford, announced the pregnancy in a statement in Wellington, saying the couple wanted a family "but weren't sure it would happen for us, which has made this news unexpected but exciting."

"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 first knew of her pregnancy on Oct 13, in the midst of talks to form a coalition government after an inconclusive general election in September. She will take a six-week break after the birth, during which time Deputy Prime Minister Winston Peters will be acting leader.

"Mr Peters and I have a great relationship, and I know that together we'll make this period work... at the end of my leave I will resume all Prime Ministerial duties," she said in an emailed statement.

Ardern said she would 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, 40, 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."

The 37-year-old Labour leader took the helm as Prime Minister in October, ending almost a decade of centre-right National Party rule. Her 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.

Late Pakistani prime minister Benazir Bhutto is believed to have been the first head of government to have given birth while in office, when she had a baby in 1990.