New Zealand's popular prime minister, John Key, announced his surprise resignation on Monday after eight years in office. He said he felt it was a "good time" to step down.

"This is the hardest decision I've ever made and I don't know what I'll do next," said Key, 55, who has led his center-right National Party for a decade. "I feel like I'm going out on top."

Key said part of the reason behind the move was to spend more time with his family, but that was just one of several factors.

"Over the years I have observed many leaders who, in a similar position, fail to take this step. I can understand why. It is a hard job to leave," he said. "But, for me and the National Party, this is a good time to go."

'I have left nothing in the tank'

A former currency trader with Merrill Lynch, Key entered parliament in 2002 and became leader of the National Party only four years later. He was voted in as prime minister in 2008, ending nine years of Labour Party rule.

Key's administration has been lauded for its handling of the 2008 financial crisis, as well as a catastrophic earthquake in Christchurch in 2011 that killed 185 people.

"All I can say is that I gave it everything I had," Key told the press. "I have left nothing in the tank."

The National Party will meet next week to appoint his successor, and Deputy Prime Minister Bill English has said he is considering running for the position. "I'll be talking to caucus and family today and tonight," said English, who also serves as finance minister, speaking to reporters in Wellington "I wouldn't stand if there wasn't strong caucus support for me standing."

'Say it ain't so, bro'

Reacting to the news, Australian Prime Minister Malcolm Turnbull said he sent Key a brief message: "Say it ain't so, bro."

After the lighthearted remark, Turnbull turned serious. "John Key is one of the most outstanding national leaders in the world today. He has done an extraordinary job for New Zealand. He is somebody that all of us, right around the world, leaders in countries large and small, draw inspiration from," said Turnbull.

es,kbd/cmk (AP, AFP, Reuters)