Bill English has been sworn in as New Zealand's prime minister, succeeding John Key who unexpectedly resigned from office last week.

Mr English, who was serving as finance minister and deputy prime minister for the ruling National Party, was sworn in at Government House in Wellington.

Social Housing Minister Paula Bennett has been named as deputy prime minister.

The appointments were confirmed after a meeting of the National Party's caucus.

In a tweet, Mr English said he was "excited and humbled" to be elected.

Image copyright Twitter

In his first speech following his election, he said he would lead a "strong team of MPs, full of ideas".

He described Ms Bennett as "smart, accomplished and energetic".

National Party President Peter Goodfellow said Mr English and Ms Bennett would be "outstanding leaders" who would "provide 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.

Mr English joined parliament in 1990 and has held several ministerial posts in education, health and finance.

Image copyright Getty Images Image caption John Key said said his job required great sacrifices "from those who are dearest to me"

Mr Key said last week his decision to stand down was for family reasons and "the hardest decision I've ever made".

He said he would not be seeking a fourth term in the 2017 election.

Mr Key tweeted soon after Monday's announcement: "Congratulations to my good friend @honbillenglish on becoming the new leader of the @nznationalparty and the next Prime Minister of NZ."

Australian PM Malcolm Turnbull also congratulated him in a phone call according to Mr English on Twitter.