Sir John Key has joined the campaign to free up the prime real estate on Auckland's waterfront.

He has joined forces with fellow former Prime Minister Helen Clark to call for the port to be relocated to Whangarei.

It is the first time the three-term leaders have publicly lined up on an issue since leaving Parliament.

Key told Newstalk ZB's Mike Hosking it made no sense to keep the port in the CBD, when there are other good ports not far away.

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He said Tauranga's port was rapidly growing and Northport could handle the extra load.

"You'd four-lane the highway to Whangarei, and you release an enormous amount of land down on the waterfront which then Auckland Council owns," Key said.

Auckland port. Photo / Jason Oxenham
Auckland port. Photo / Jason Oxenham

Key said selling the land would scratch an "enormous" amount of debt off the council's balance sheet, and allow the waterfront to be used far more effectively.

"World class cities with a waterfront like we've got, don't lock it away with big red gates and take hundreds of trucks through town every day."

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Key said some estimates put moving the port at $10 billion, while others say the cost of keeping it in the CBD tallies up to $8b.

"The differential between the two is not insubstantial - probably a few billion - but not nearly as big a number I think as some people say."

Speaking on the AM Show, he said moving the port would release an enormous amount of land down on Auckland's waterfront, creating an opportunity for Auckland to get more money and reduce its borrowings.

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"It could use that money for a tonne of other initiatives it's got," he said.

He also said people needed to think about the long-term benefits which could include a national waterfront stadium as well as other options.

"You look back in 10, 15, 20 years' time and say what a magnificent waterfront Auckland has," he told the AM show.

The campaign lines up with the Government's working group on the future of upper North Island ports, which concluded earlier this month that Auckland's port should be moved to Northland.