New Zealand prime minister Jacinda Ardern is expecting her first child with her partner Clarke Gayford, she announced in a Facebook post.

"Clarke and I are really excited that in June our team will expand from two to three, and that we'll be joining the many parents out there who wear two hats," the Labour PM wrote.

"I'll be prime minister and a mum, and Clarke will be 'first man of fishing' and stay at home dad. I think it's fair to say that this will be a wee one that a village will raise, but we couldn't be more excited."

Ardern, who took office in October, said that deputy prime minister Winston Peters will take her place for six weeks when the baby is born in June. She added she intended to be "fully contactable" during her maternity leave and that she intended to resume "all prime ministerial duties" once her leave ends.

The prime minister's announcement was met with good wishes from at least two former New Zealand prime ministers. Former prime minister Jenny Shipley called the news "brilliant," telling RNZ: "It's a wonderful choice of our outstanding young woman leader who is also going to choose to be a parent.

Shipley added: "It will bring special insight I suspect in her work and joy to her family." Fellow former prime ministers Helen Clark and Bill English shared their best wishes on Twitter.

Malcolm Turnbull, Australia's prime minister, called Ardern to congratulate her and shared his best wishes on Twitter. "Congratulations @jacindaardern on your wonderful news today. When we spoke this morning you sounded more excited than you did when you won the election! Lots of love and best wishes from me and Lucy and all of us across the ditch," Turnbull tweeted.

US Ambassador Scott Brown also congratulated the couple on the news.

Ardern, 37, became New Zealand's youngest prime minister since 1856 in October after forming a centre-left coalition. She learned of her pregnancy just six days after learning she would be prime minister.

"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 noted.

New Zealand's prime minister is not the first female world leader to give birth while serving. In 1990, Pakistan's Benazir Bhutto gave birth to a daughter.

Jacinda Ardern
New Zealand's opposition Labour party leader, Jacinda Ardern, speaks during an event held ahead of the national election at the Te Papa Museum in Wellington, New Zealand 23 August 2017. Reuters