The best piece of advice former Prime Minister Sir John Key gives to Winston Peters as he prepares to become Acting Prime Minister is "Enjoy it – it's the best job in the world."

"I think for six weeks he will thoroughly enjoy being in the role," Sir John told the Weekend Herald.

"I mean he has dreamed about being in the role for a very long time."

Jacinda Ardern's due date is tomorrow, June 17, and the plan is for Peters to step up from the deputy's job for six weeks.

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Key said Peters could not afford to have sour relations with the news media, which has been a hallmark of his career.

"The Prime Minister is the person who is the face and voice of the Government and if they look frazzled and under pressure, then that is just a proxy for the Government being under pressure," said Key.

"For a long time in his career, he has turned the story into one of him versus the media but that is not a luxury that the Prime Minister of the day really has, not if they want to be effective," said Key.

"I think for Winston this is a real opportunity for him to demonstrate that he has got a skill set wider than just barking at the media because it is not about the media. It is about the country, and the media is the vehicle for transmitting those message of whatever the message of the day is to the people.

"Good Prime Ministers know that and so they have to keep that temperament going."

Key described a demanding job in which he got up at 5.40am and tried to get to bed by midnight.

Large chunks of time were pencilled out for pre-Cabinet briefings, Cabinet, Cabinet committees, caucus, Question Time, pre-Question Time briefings, and evening functions.

He had been delivered a briefcase every day with readings in it, some needing immediate attention, others for background.

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"Put all of that together as well as the general management, plus the media time it takes to make sure you are on top of every issue and very quickly you can see that your spare time is next to nil."

The only guaranteed time off was Sunday and that was also taken up with reading Cabinet documents and talking to ministers.

For the most part Prime Ministers were people who interacted well with people.

"You can see that with Jacinda," he said. "She was at the Fieldays and she's potentially days away from having a child and she's walking around the Fieldays. It is pretty brave of her to do that. It's a tough ask to be doing that so close to having a baby."