Prime Minster Jacinda Ardern has gone into hospital to give birth, her office says.

New Zealand First leader and deputy Prime Minister Winston Peters is now acting Prime Minister.

Ms Ardern has previously said she would take six weeks maternity leave.

Her due date was Sunday 17 June.

RNZ's live updates here, and read Jane Patterson's explainer of Mr Peter's duties as Acting Prime Minister

She and partner Clarke Gayford arrived at hospital at 5.50am.

Mr Peters said said he was advised this morning that the prime minister had been admitted to hospital.

"It's a happy day and on behalf of the coalition government we wish her and Clarke all the very best."

The Green Party co-leaders have send their best wishes, saying it is historic moment for the country.

The deputy leader of the National Party, Paula Bennett, wished Ms Ardern well in Parliament this morning.

It will be Ms Ardern's first child, and Mr Gayford will be primary caregiver.

Ms Ardern found out about the pregnancy last October and told Mr Gayford about the news through Facebook video call. The couple announced the pregnancy to the public on 19 January.

Last Monday, Ms Ardern was advised not to fly any more and since then had been working from home in Auckland, rather than travelling to Wellington. Mr Peters carried out some of the prime minister's Wellington-based duties this week, chairing Cabinet and hosted Ms Ardern's weekly post-cabinet media conference.

Yesterday Clarke Gayford tweeted a photo of the prime minister still working at home.

Anyone with aspirations of being PM, make sure you also have an appetite for reading and reading and reading and reading. #stillwaiting pic.twitter.com/AzwzkYPIKD

— Clarke Gayford (@NZClarke) June 20, 2018

There won't be any more formal announcements from the Prime Minister's Office until Ms Ardern and Mr Gayford announce the birth - likely on social media. The couple have yet to reveal the baby's gender, but Ms Ardern was gifted the name Waru at Ratana celebrations and the name Waimirirangi was offered at Waitangi Day celebrations if the baby was a girl.

If the prime minister's baby is born today he or she will share a birthday with Pakistan's former Prime Minister Benazir Bhutto, who is the only other elected leader to have given birth while in the role.

The much-anticipated arrival has gone global, as international media outlets keep track of what is unfolding at Auckland Hospital.

The New York Times reports Ms Ardern's pregnancy has prompted a national conversation about working mothers.

Jacinda Ardern, New Zealand's Prime Minister, is expected to have her baby soon https://t.co/uxNSx9tNMz pic.twitter.com/U2Pt7OFMjC

— New York Times World (@nytimesworld) June 20, 2018

Outside the hospital where the PM is in labour #firstbaby pic.twitter.com/CLyX4oGfe5

— Jessie Chiang (@je_chiang) June 20, 2018