It's just two months since John Key resigned and it feels like a year. The shock passed so quickly that many now might deny they felt it.

Key says it was the hardest decision he has made. It could also be the best.

I confess I didn't think so that unforgettable Monday, December 5. When the news first came through I thought his decision reckless, cavalier, selfish. I worried for the country.

I knew I was by no means alone in the conviction that a great deal of the economic stability and growth that New Zealand, almost alone in the world, was enjoying, rested on business confidence in John Key.

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I was at lunch in Wellington that day with senior newspaper people and others from different walks of life, and the shock was palpable. Initial disbelief gave way to stunned quiet, punctuated by dark humour and wild conjecture as we awaited word from his press conference.

I can't say I felt much better when he read out his written explanation.

Why couldn't he "look down the barrel" of cameras and say he would serve another three years? Why did he suppose the country would turn against him at some point in a fourth term? When a government is going out it is normally evident in the second year of its third term and there was no sign of it.

Why go? Why would he pass up the chance to carve his name on New Zealand history as one of our longest-serving prime ministers, not just beating Holyoake's 11-year run but, the way his popularity was holding, it was conceivable he could match Massey, maybe even Seddon for longevity.

If that didn't matter to him, what about the interests of the country?

Then something remarkable happened. In two days I was over it. His decision felt right. Again it felt like I was not alone. Bill English would be different and interesting, the country would be fine. It already felt stronger for the discovery it was not dependent on a prime minister.

Key's final decision was such a good one that many might deny now they were ever worried.

But everyone in politics or close to it knows the decision sets him apart from all other prime ministers in our experience who have outstayed their public regard. Key's decision is not just unprecedented, it is foreign to the culture of politics.

It comes, I think, from corporate life. Those of us who don't work in executive suites are often surprised that chief executives step down when their companies are doing well.

When Key agreed to be interviewed for an update of Portrait of a Prime Minister I was about to hear how somebody quite unlike me can let go of a job he loves because he has decided logically and objectively the time was right - for him, for a successor, for continuity.

We talked at his home the morning after he left office, eight days after his announcement.

He wanted to leave well, and the way he went about is, I think, worth knowing.

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He and Bronagh were preparing to fly out to Hawaii that night. He made us a cup of tea in the kitchen. Bronagh, chatting happily at the breakfast bar, talked about her ageing father who would be going with them for Christmas.

I should have known better than anyone outside his family and staff that he could walk away at any time.

When I'd last talked to them both, in the summer of 2014 for the first edition of the book, they told me late in the session about a conversation they had in Hawaii two years earlier. Key called it "kicking the tyres". The tyres of retirement - after just four years.

He regretted mentioning it and when the book came out he told a press conference the chat had been entirely for his wife's sake. But that hadn't been the way it came across.

If anything, Bronagh had sounded more definite than him that departure at that point would feel like failure.

Nobody could call it failure now. Key describes the decision as the hardest of his life. He loved being prime minister, relished the work, the contact with people, the myriad decisions demanded of you daily and even the continual, often petty, cut and thrust of politics.

It is the last-mentioned that usually discourages top people in the private sector from standing for Parliament.

Key was a rare exception but he did not cease to approach his job like a chief executive, especially when it came to leaving it. He did not want to leave rejected and embittered like those before him.

He wanted to leave well, and the way he went about is, I think, worth knowing.

• The updated edition of Portrait of a Prime Minister, published by Penguin Random House, was released yesterday.