New Zealand Prime Minister Jacinda Ardern has announced she is pregnant and will "be PM and a mum".

Key points: Jacinda Ardern's first child is due in June

Jacinda Ardern's first child is due in June She has previously been questioned about juggling motherhood and her career

She has previously been questioned about juggling motherhood and her career She has been serving as New Zealand's Prime Minister since October

Her news comes after she was quizzed about motherhood when she was first elected as the Labour Party leader.

She posted the announcement on Twitter, saying she will "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," she wrote, adding that she expected a lot of questions.

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She was elected as the Labour Party leader in August and was in the job less than 24 hours when she was asked about balancing her career and potential motherhood.

At the time, the host of The Project in New Zealand, Jesse Mulligan, asked if she felt she needed to make the choice between motherhood and her career.

Despite public criticism of the question, Ms Ardern said she had "no problem" answering it.

"I've been really open about that dilemma because I think probably lots of women face it," she said.

"For me, my position is no different to the woman who works three jobs, or who might be in a position where they are juggling lots of responsibilities.

"You've got to take every day as it comes and try and see if you can make the best of the lot you're given.

"So I'm not pre-determining any of that, just like most of the women out here who just make their lives work."

Following her comments about motherhood, some asked whether it was OK for her to take maternity leave while in office.

While she answered questions, Ms Ardern said it was "totally unacceptable" to ask women that question in the workplace.

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Ms Ardern said deputy Prime Minister Winston Peters would serve as acting Prime Minister for six weeks after her baby was born.

"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.

Jacinda Ardern and her partner Clarke Gayford are expecting their first child in June. ( AAP: David Rowland )

Mr Peters told the ABC he thought New Zealanders would be supportive of the Prime Minister's announcement.

"I think the country is pleasantly bewildered and excited by it," he said.

"It will be regarded as being unusual but that maybe is because some people have a chauvinistic attitude towards society. We live in a different age now."

'We consider ourselves to be very lucky'

Ms Ardern said she and her partner were both happy about the news.

She found out about her pregnancy in October, six days before she was elected Prime Minister.

"I felt like we set a high bar for juggling a range of issues early on. You had to get on with it," she said.

"Lots of people deal with things in their personal lives and things going on in their work lives. I was just doing it in a public way."

Ms Ardern said once she returns to work, her partner Clarke Gayford would stay at home and care for the baby.

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

"We consider ourselves lucky for another reason. Clarke and I have always been clear we wanted to be parents but had been told we would need help for that to happen. That's made this news a fantastic surprise.

"We first knew of my pregnancy on the 13th of October, but as many couples do in the early stages, we kept it to ourselves.

"From a personal perspective, I am so looking forward to my new role as a parent. But I am equally focused on my job and responsibilities as Prime Minister.

"While 2018 will be the year I become a mum, it will also be a year that the Government finishes our 100-day plan, and starts pursuing all of the priorities that will build a better New Zealand. I look forward to leading that work, and having a slightly expanded family join me on that journey. "

New Zealand Labour Party leader Jacinda Ardern was previously quizzed about motherhood. ( AP: Mark Baker, file )

Ms Ardern said she was not the first woman to multi-task or 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.

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

She confessed she knew the gender of her baby, but said it was a secret between her and her partner.

Ms Ardern said her pregnancy would not be the only milestone for government this year.

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

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ABC/Reuters