When John Key became our 38th Prime Minister on November 8, 2008 it was an extraordinary turnaround from the previous election.

In 2005, New Zealand chose Labour with two seats more than National. Three years later, National took the helm with 58 seats to Labour's 43 and Key's leadership has kept the country wrapped in a blue ribbon ever since.

Whether he has ridden a wave of popularist middle-ground politics or he built that wave will be better left to a day much later to judge.

As is the world we live in, there are any number of fanciful theories behind Key's surprise announcement. The simple fact is, the job of Prime Minister is extraordinarily stressful.

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Key cited family reasons for leaving, saying the job had required great sacrifices "from those who are dearest to me". His wife Bronagh had endured "many lonely nights" and his children Stephie and Max had been put under "extraordinary levels of intrusion".

On Monday, John Key will have been Prime Minister for 2956 days. This is not the longest by any stretch. Richard Seddon held the position for 13 years between 1893 and 1906.

Asked about his next step, Key said he was "a commercial guy" and was likely to take up board positions, possibly with companies in Australia.

Unlike predecessor Helen Clark, he has no interest in international politics or a United Nations job.

Asked what his legacy would be, Key said stabilising and growing New Zealand's economy after the global financial crisis and weathering crises such as the Canterbury earthquakes.

His main regrets were failing to ratify the Trans Pacific Partnership, not getting the Kermadec Ocean Sanctuary opened, and not changing the national flag.

His page on the National Party website this afternoon bore the message: "I want to leave New Zealand in better shape than I found it. I know the job of Prime Minister is not forever and I'm going to do the best I can every day to make that difference."

Key often began his remarks with the phrase "at the end of the day". At the end of Monday, December 12, Key will be former prime minister.

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At the end of the day, it has been an extraordinary run.

- Edward Rooney is the Regional News Editor at NZME News Service.