Former Prime Minister John Key on Helen Clark's unwitting role in his resignation - and son Max's reaction

Key Points:

Key says talk with Clark helped him decide to quit at the top of his game.

Former Prime Minister John Key says a conversation with Helen Clark helped make his decision to retire while at the top of his game - but admits it was the hardest decision of his life and he did wake up in the night wondering about it.

An updated edition of John Roughan's John Key: Portrait of a Prime Minister includes further material from the final years of Key's time as Prime Minister as well as a post-resignation interview.

In it, Key says he made the final decision after a trip to New York last September, during which he had also had a conversation with Clark, who left Parliament after she was defeated by Key's National Party in 2008.

Key says that in the conversation, he and Clark discussed the right time for a Prime Minister to resign. He did not believe she realised the significance of the conversation at the time, saying it was "a conversation she didn't know she was having".

He does not specify what Clark said - and Clark did not respond to questions about it.

However, Key has since said part of the reason was he did not want to be unpopular.

Key says it was the hardest decision of his life, but he was still certain he made the right call. Despite that, he admitted he had woken up at night sometimes worrying about whether he had let down the public or whether National felt he had scuppered their chances of a fourth term.

However, he remained adamant he had made the right decision, saying to leave it longer would have meant a "hospital pass" for English.

"I absolutely understood the gravity of what I was doing, but even though I went round in circles a bit, I thought 'this feels right and in the end I've got to trust my instincts."

Key also has a prediction for the 2017 election: that Bill English will get that fourth term - and that NZ First leader Winston Peters will side with National over Labour if he has the balance of power.

In the book, Key also reveals son Max did not believe he should step down as Prime Minister at first and tried to talk him out of it.

The book says his daughter Stephie took the news Key would resign well - she had moved to Paris partly to escape the attention.

However, Max was another matter.

Key said it was not until Max saw the public's reaction to Key at the Joseph Parker boxing match on the night of his resignation that he told Key he had done the right thing. Key also opens up about the impact his job had on Max, who was 11 when Key became PM.

His proudest memory was of Max saying he believed it was a good thing he had done it.

Key also reveals he alone had made the decisions to deploy the SAS back to Afghanistan and defended his anger at Labour leader Andrew Little for his opposition to the Iraq deployment which prompted Key to shout "get some guts" at Little in Parliament.

The book also looks at the problems that assailed Key and his government in the last few years - including putting $11 million into an agrihub in Saudi Arabia to try to secure a free trade agreement, and Key's actions pulling a waitress' pony tail.

The book says that Key "several times suppressed the urge to tell his side of the story" when asked about pulling the ponytail of a waitress at Rosie cafe in Parnell, who blew the whistle about the Prime Minister's behaviour.

Key says what happened was "not the way it was portrayed" but he had decided not to try to defend himself over it.

"Well, there's so much stuff I could go into but I decided I'm not going to. But I should have picked up on it. You know when something starts as a joke and carries on and goes on too long. That was it."

Key was back in Parliament this week but told Roughan he had signed up for the international speaking circuit, was likely to get back into investment banking and was likely to turn down a request to be on the board of an American airline: "It would feel a bit like coaching the Wallabies."