Jacinda Ardern has been officially sworn in as Prime Minister and told a boisterous crowd at Parliament that she will lead a strong, empathetic and inclusive Government that should be judged on how it treats the most vulnerable.

Ardern was sworn in at Government House this morning during a formal ceremony, attended by Ardern's partner Clarke Gayford and her father Ross.

Ardern has another reason to celebrate today - her sister Louise has had a baby boy. The birth was the reason her mother Laurell was not at the ceremony at Government House.

Ardern confirmed to Governor-General Dame Patsy Reddy that she is able to command a majority in the House of Representatives, and Dame Patsy then signed her prime ministerial warrant, making her the country's 40th leader, to a roomful of loud applause.

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The 31 ministers and under-secretaries then took the oaths of allegiance to Queen Elizabeth II.

Meanwhile a 600-strong crowd had gathered on the front lawn of Parliament by the time the ministers' bus arrived back from Government House.

Among them was a band of four, calling themselves Stardust Stompers, who wanted to support the new Government with some "groovy tunes", said frontman Joe Lindsay, a member of Fat Freddy's Drop.

"I think she's great, and I think it's gonna be great for New Zealand," Lindsay said.

Deborah Read, from Otaki, brought two red roses - one for Ardern and one for Winston Peters. She had told Peters on his Facebook page to choose Labour, and she was glad he had followed her advice because they complemented each other well.

Loud cheers erupted when Ardern stepped out, followed by her ministers and under-secretaries.

She joked that it was great to have "roughly 1/8 of the band Fat Freddy's Drop" playing at what amounted to a public inauguration.

It was "an enormous privilege and an honour" to serve New Zealand, she told the crowd.

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"We take incredibly seriously the roles and responsibilities that we have adopted today.

"Not everyone voted for us ... But we vow that regardless of who you voted for, regardless of where in Aotearoa New Zealand you live, this will be a Government for all New Zealanders.

"We will be an active Government. We will be a strong Government. We will be an empathetic Government. And we will be a Government that works together."

Ardern said New Zealand was a wonderful place "that can be even better", where everyone deserves decent wages, world class healthcare and education, and a healthy environment.

"We are a society that should be judged by how we look after our most vulnerable."

She said the Government had a lot to do. "And we cannot do it alone ... We will work with the business communities, we will work with NGOs and community organisations, we'll work with Maori ... we will work together. And that work begins today."

Ardern's new Cabinet is meeting for the first time at 3pm.

"There will be good days and there will be bad days, but no matter what, we will always remember the vote that you gave us, the vote of support, and the debt of gratitude that we owe you.

"So ladies and gentlemen, without further ado, let's go and do this."

Ardern then walked along the front of the forecourt, greeting fervent supporters amid "Jacinda" and "Labour" chants. She then embraced Gayford and his two nieces and, hand in hand, walked up the steps to Parliament House.