New Zealand's socially conservative finance chief Bill English was sworn in as the country's new Prime Minister on Monday following last week's shock resignation of his popular predecessor John Key.

The centre-right National Party caucus unanimously backed English at a meeting on Monday morning and he travelled to Government House in Wellington a few hours later to officially take over.

State Services Minister Paula Bennett was named as deputy leader.

Mr. English, 54, said he was "excited and humbled" to take the top job after eight years as Mr. Key's deputy and Finance Minister.

"This will be a government supporting economic growth and ensuring that the benefits of growth are widely shared," he told reporters.

National Party president Peter Goodfellow said Mr. English and Ms. Bennett offered "a good mixture of experience and fresh thinking".

"Under their leadership, New Zealanders will continue to benefit from the stable government they expect, along with a dedicated focus on delivering results for families and businesses," he said.

A former farmer with degrees in commerce and literature, Mr. English has been in parliament since 1990 and was previously leader of the National Party in 2002 when it suffered its worst election defeat.

"You learn more from losing than you do from winning," said Mr. English, who will seek National's fourth straight election win in late 2017.

He was Mr. Key's preferred successor after returning New Zealand's budget to surplus and keeping the economy ticking over at about three percent.

Social conservative

Mr. English said New Zealand's prosperity meant the country did not have the pool of disaffected voters responsible for Brexit and U.S. President-elect Donald Trump's victory.

And he said a priority for his government was ensuring the most needy were given opportunities.

"We have a strong economy, almost unique in the developed world, and most New Zealanders would expect to be able to share in that," he said.

A committed Catholic with six children, Mr. English is regarded as far more socially conservative than Mr. Key, opposing the 2013 legalisation of same-sex marriage and speaking out against abortion and voluntary euthanasia.

"It doesn't define me but it is an important influence," he said when asked about his faith on Monday, adding that he now supported gay marriage after seeing its positive impact.

Mr. Key, who resigned for family reasons after eight years and Prime Minister and 10 as party leader, said he was looking forward to becoming an anonymous backbencher.

He congratulated Mr. English and Bennett, saying he did not expect the government's direction to change under the new team.

"I don't think it will be a radically different agenda under Bill English," he told reporters.

"It gives a sense of newness (to the government) that the public probably do want."

Opposition Labour Party leader Andrew Little said Mr. English's leadership meant more of the same for voters.

"New Zealand has moved on, but Bill English hasn't," he said. "The right-wing rump of National under English is now reasserting itself."

Ms. Bennett, 47, revealed she had struggled as a teenage single mother and said the fact that she was given a second chance and had become Deputy Prime Minister was "a credit to New Zealand".

"There was a moment when I was a 17-year-old Maori solo mum in Taupo, I'd left school with no qualifications, I didn't have a job and it looked pretty bleak," she said.