Prime Minister Jacinda Ardern is pregnant, but she's reluctant to be called a trailblazer, saying she's going to be far from the first woman to balance work and children.

Ms Ardern, 37, on Friday announced she and her partner, television presenter Clarke Gayford, were expecting a baby in June, describing it as a "surprise".

Deputy Prime Minister Winston Peters will take on the top job for six weeks after the baby is born.

"He's played that role before. When I go overseas he plays that role. That's not unusual," Ms Ardern told reporters outside her Point Chevalier home.

"After those six weeks, I'll be coming back on deck."

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While acknowledged she faced a challenge balancing home life and her prime ministerial duties, Ms Ardern was reluctant to paint herself as a
trailblazer.

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

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"I am about to sympathise with them a lot, as I sympathise with all women who suffered morning sickness.

"And New Zealand is going to help us raise our first child."

Ms Ardern said she received the news on October 13 - in the midst of negotiations over who would form the new government, but had kept it quiet - common in the early stages of pregnancy.

While saying the news didn't affect the talks, she admitted it weighed on her.

"I am a human. Whose mind wouldn't it play on?"

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The couple had earlier been told they would need help conceiving and had stopped getting assistance when Ms Ardern became Labour leader last August, she said.

Mr Gayford - who the Prime Minister accidentally referred to as "Dad" during Friday's press conference - would be primary carer and would travel with her as much as possible.

"Knowing that so many parents juggle the care of their new babies, we consider ourselves to be very lucky," she said.

She declined to reveal the baby's gender and said no name had been chosen but said it would likely have Mr Gayford's surname.

In a statement, NZ First leader Mr Peters voiced his support.

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"Ours is a novel coalition in many respects and the Prime Minister's announcement only reinforces its unique qualities," he said.

Ms Ardern and Mr Gayford received congratulations from across the political spectrum following the announcement - including from former prime ministers Bill English, Helen Clark and Jenny Shipley - and prompted more than 40,000 tweets in the first hour.

World leaders, including Australian PM Malcolm Turnbull, also sent their good wishes.

Ms Ardern is not the first world leader to be pregnant. Pakistan's Benazir Bhutto caused a stir with her second pregnancy in 1990.

Hours after taking the Labour leadership last year, Ms Ardern was probed by television presenter Mark Richardson about whether she was considering having children, responding that the question was "unacceptable" and prompting a debate about women's workplace rights.