Prime Minister Jacinda Ardern has gone into hospital to give birth and was driven there by partner Clarke Gayford this morning.

Photo: Katie Scotcher

The Prime Minister's office said Ms Ardern was driven to hospital by her partner Clark Gayford. They arrived at 5.50am.

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 Peters' duties as Acting Prime Minister

Mr Peters 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."

However, he played down what is a historic day, with his promotion to head of the country and the impending arrival of the Prime Minister's baby.

Mr Peters said when people are around politics as long as he has, nothing was unusual, but he wished the first couple well.

"I would hope that the people of this country, first of all wish the prime minister the very best ... and the she gets a solid start to her motherhood so to speak."

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.

Staff at the hospital have also been eagerly awaiting the birth.

Tanisha Jowsey - who works on the 12th floor - said all her co-workers were very excited.

"We've been waiting for this day for like the last week. We've been looking every times there's any media around, going 'oh maybe today's the day'.

"We've got quite a few spotters out and about throughout the wards just keeping an eye on things."

Ms Jowsey said Ms Ardern seemed fit and healthy so hopefully the baby will arrive soon.

Outside the hospital, people have been guessing the baby's sex and name.

"A girl, definitely a girl. Girl Power," one of the women guessing outside said.

"I got a feeling, I reckon it's going to be a little lad ... be fishing with Clarke in a few months' time in a front pack and yeah, look forward to it," another said.

"I would hope it's a girl. I would hope she follows her mum's footsteps if it's a girl, yeah, that would be great," a woman outside the hospital said.

Those guessing the name had some humour about it.

"Christina because it's a famous name, so yeah, might as well."

"Uh, maybe Winston?" another said.

Already 16 babies have been born today at the hospital, but it's not clear whether that includes the Prime Minister's.

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.

Photo: Instagram / Jacida Ardern

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