Prime Minister John Key, who's just returned from a trip that took in post-referendum UK, says the country is in a "real mess".
Last month British voters narrowly elected to leave the European Union (EU), an unexpected result which sent markets reeling.
Mr Key was the last world leader to visit David Cameron while he was still Prime Minister.
"It was quite sad in a lot of ways," Mr Key told Paul Henry on Monday morning.
"He was in jeans and a polo shirt packing boxes. They grabbed Nancy, their daughter - who'd been on a school trip - back from France, because they didn't want her to hear it from the media. He was then having to do everything, including prepare for Question Time the next day."
Mr Key says Mr Cameron told him a lot, much of which was between "mates" and he wouldn't repeat. But he did say Mr Cameron was adamant UK voters had made a mistake in choosing to leave the EU.
"He's leaving at a time where he feels after six years in the job, Britain's going to a place he just doesn't think it shouldn't be going."
While there, Mr Key spoke with "everyone from major business leaders right through to the Governor of the Bank of England", none of whom had a clue about what would happen next.
"Their big problem is when they invoke this Article 50 - that's the divorce proceedings, and they have two years to put all that together - they can't negotiate a free trade agreement until they get divorced. No member state - and they're still a member state of the EU - can do that.
"This is where their real problem comes in. Negotiating an FTA might take five years, and while they're doing that the WTO rules apply, which means all the manufacturing that happens in the UK will be at a huge disadvantage - in some cases 20 percent tariffs to Europe. It's a real mess."
But why did Mr Cameron let the referendum go ahead? Mr Key says Mr Cameron thought it was inevitable, and wanted to make sure a pro-EU person like himself was in charge when it happened.
"The theme in the editorials in the newspapers in the UK and internationally was that he was too cavalier in allowing this referendum - his perspective on that would be to say that's not right," says Mr Key.
"In 2013, the Tory party come hell or high water was going to have this referendum, and he wanted to make sure if they had the referendum there was a Remainer leading the Government and leading the Tory party, not a Brexiteer."
Mr Cameron has been succeeded by Theresa May. She was anti-Brexit - not as strongly as Mr Cameron however, and has promised to follow through with Article 50.