Former Prime Minister Sir John Key once helped the future King, Prince William, dodge his security detail to go for a walk alone on his visit to New Zealand in 2011.
The incident is outlined by Key in a new book, Queen of the World, by Daily Mail columnist Robert Hardman, which looks at the Queen's work around the world.
The book also reveals just how nervous Buckingham Palace was about New Zealand becoming a republic in the 1990s, and includes de-classified material from Britain's Foreign Office about the effect Britain entering the European Union had on relations with New Zealand.
Key told Hardman that on Prince William's visit to New Zealand in 2011 after the Christchurch earthquakes and Pike River mining disaster, Key and William were in rooms next to each in a hotel on the West Coast.
"In the morning, I was writing my speech on my balcony and he said to me from his balcony: "Do you think I could go for a walk – on my own?"
I said: "Go that way".
And he climbed down and off he went.
That's the nice thing about New Zealand – you could do that.'
Key told the Weekend Herald security never found out about the unauthorised walk.
In the book, Key also described the Queen as the most impressive person he had ever met, and recounted asking her why she still wore formal dress on occasions when there were no crowds or cameras around.
"I am the last bastion of standards," she replied.
The book also reveals that in the 1990s, while Australia was preparing for its 1999 referendum on a Republic, the Palace was worried about the domino effect on New Zealand.
"An ex-member of the royal team at the time says that they had explored and 'road-tested' every scenario.
"There was a fear of a run on the Crown. We expected the outcome would be okay, given the threshold, but the risk was that it would trigger something in a country like New Zealand where the result just needs to be fifty per cent plus one. So there was concern about the domino effect."
The same source also claimed the Palace came up with a rule that any country that did opt to change could not delay it until after the reign of Queen Elizabeth, saying otherwise there would be a "death watch" scenario.
Politicians in both New Zealand and Australia have said they expected republicanism to become a live issue after the death of the Queen.
The book points to subsequent royal marriages and births as a significant factor in putting republican sentiment on the backburner in both New Zealand and Australia – from Charles and Diana to Princes William and Harry, who is due to visit again this month with his new wife Meghan Markle.
It said a succession of visits by younger Royals meant republican sentiment in New Zealand now was "dormant, if not comatose."
The book also details how New Zealand delivered two 'firsts' to Royal life – the tradition of the 'walkabout' with members of the public and commercial flights.
The first 'walkabout' was in Wellington on the Queen's New Zealand tour in 1970 when she agreed to break protocol by getting out of her car and meeting members of the waiting public.
The Queen's first commercial flight was on Air NZ in 1995 after then Prime Minister Jim Bolger suggested she fly commercially – apparently to save money.
The Queen flew from London via Los Angeles on NZ1, watched a Sam Neill movie and had First Class all to herself.
Her entourage were in Business and ordinary passengers – including Harding as travelling media – were in economy.
All got a commemorative pen, which Hardman said he still has.