Sir John Key says if Auckland's port is moved northward, the city might be able to finally get its waterfront stadium.
"If you were designing Auckland today, would you put the port there? The answer is no," Sir John told The AM Show on Monday.
"There's a little bit of cost involved here, but if you could do it, you would release an enormous amount of land down on Auckland's waterfront. What that would mean is... Auckland would get a lot of cash for that, many billions of dollars, which it could use ultimately to reduce its borrowings at the moment. It could use that money for a tonne of other initiatives it's got."
While the cost of moving the port to Northland has been estimated between $8 billion and $10 billion, Sir John says people need to think longer-term.
"You've got a spectacular opportunity down there. You look back in 10, 15, 20 years' time and say what a magnificent waterfront Auckland has."
And that could include the long-fabled waterfront stadium.
"All those options become available to you. I personally quite like the idea."
In 2016 as Prime Minister he ruled out Government funding of a waterfront stadium, estimated to cost potentially $1 billion. But freeing up the land could see Auckland Council get a potential $8 billion cash injection, Sir John said.
But an economist is rubbishing the plan, saying there's an obvious location for New Zealand's primary port that makes much more sense.
"There's room for one super-size port in New Zealand - it's got to be Tauranga," Cameron Bagrie told The AM Show.
As well as Sir John, former Prime Minister Helen Clark has thrown her weight behind the Waterfront 2029 campaign to relocate it to Marsden Pt.
She said it would provide development and employment benefits for Northland. New Zealand First has long pushed for the idea too.
But Bagrie says it makes no sense.
"We're going to be shifting goods 150km north of Auckland, back into Auckland. And a lot of stuff that comes into Auckland gets redistributed south into Hamilton, Tauranga."
He said it's "obvious" Tauranga needs beefing up and Auckland scaling back. Sir John agrees, but thinks a move north - though costly - will be worth it.
The head of Ports of Auckland is also against the move, telling Stuff the pro-move campaign was using a "jumble of made-up 'facts'" to back their push.