New Zealand Prime Minister Jacinda Ardern has announced she is pregnant with her first child.

It prompted an outpouring of support from women's rights groups and labour activists as she declared "I'll be a prime minister and a mum".

Ms Ardern said she planned to work until the end of her pregnancy in June and then take leave for six weeks, during which time Deputy Prime Minister Winston Peters will take over.

Speaking to reporters outside her Auckland home, Ms Ardern said her partner Clarke Gayford would care for the "surprise" addition full-time and that the whole family would travel together when necessary.

"I am not the first woman to work and have a baby. I know these are special circumstances but there are many women who have done it well before I have," she said.

The 37-year-old's pregnancy is one of the very few examples of an elected leader holding office while pregnant and the first in New Zealand's history.

Pakistan's Benazir Bhutto gave birth while she was prime minister in 1990.

Ms Ardern came to power through a coalition deal after a closely fought election last year.