Billionaire businessman Sir Richard Branson told Newshub New Zealand farmers should grow cannabis instead of expanding dairy operations.

He also called US President Donald Trump an "embarrassment", before considering a future job opportunity for former Prime Minister John Key.

The entrepreneur jetted in and out of the country over the weekend to speak at a charity fundraiser.

A ticket to the gala dinner would set you back $1000 but for that price, guests heard the entrepreneur's views on subjects such as marijuana, which he predicts will be as acceptable as wine in 10 years.

He also weighed in on the war on drugs.

"We've done a lot of studies on the war on drugs and it's been an abject failure, and what is absolutely clear to us is that drugs should be decriminalised and people who have drug problems should be helped," Sir Richard said.

The entrepreneur sees a big business opportunity in New Zealand for marijuana - it could allow Kiwi farmers a new venture, cannabis rather than cows.

"You should legalise it, grow it, tax it, regulate it," he told Newshub.

"I think that would be wonderful because obviously the amount of dairy cows that New Zealand has is damaging the rivers, if you could put some of that land over into growing cannabis would be just as profitable for them, if not more profitable." 

When Newshub asked Federated Farmers what they thought of Sir Richard's idea to choose cannabis over cows, national president William Rolleston wasn't completely opposed.

"Farmers welcome any opportunity to add another string to their bow, and would look at that option only if it was legal and profitable to do so... But that is a long way down the track," he said.

Sir Richard also waded into the debate over whether water bottled and sold for export should be taxed.

"Why not tax it and put that tax towards good use? If it's being sold domestically I would say no, but if it's being sold for export, a small something makes sense."

Sir Richard, whose net worth is said to be $4.9 billion, is also scathing of fellow magnate Donald Trump.

"I think Trump is the one business person I know who is certainly not the right person to be running a great country like America," he told Newshub.

"The first days in office have been so disastrous I think the chances of [Mr Trump] lasting more than one term is extremely unlikely, and I think that's the only saving grace about the way he's behaving at the moment."

Back in New Zealand, Sir Richard says Mr Key's decision to step away from power put him in good company, alongside anti-apartheid revolutionary Nelson Mandela.

"Nelson Mandela did, John Key's done, there's very few others, most of them carry on until they are no longer wanted," he told Newshub.

Mr Key joked about his unemployment at the fundraiser at the weekend, which he also spoke at, but Sir Richard doesn't think Mr Key will be jobless for long.

"I think he's in a very strong position to almost do anything he wants on the world stage, maybe I can think of some ideas to maybe put him to work," he said.

And you'd have to think, having partied with the Sex Pistols and kite-surfed with former US President Barack Obama, Sir Richard would be a fun guy to work for.