Sir John Key and Helen Clark have joined forces.

In a move designed to put pressure on the Government, the National-led Opposition and Auckland Council, the former Prime Ministers from different sides of the political divide have combined to promote the idea the Auckland port should be moved to Northland, the New Zealand Herald reports.

John Key.

The campaign follows the announcement this month that the Government's working group on the future of the ports and freight in the upper North Island has submitted its third and final report. It calls the Auckland port operations unsustainable and says they should be moved to Northport, at Marsden Pt near Whangārei.

Sir John told Mike Hosking it doesn't make sense to keep the port in the CBD when there are other very good ports not far away.

"Okay, it's very expensive to move it, but the problem with that argument is that it's also very expensive to maintain it. The differential between the two is not nearly as big a number, I think, as what people say."

He says Tauranga's port is rapidly growing, and Northport can handle the extra load.

"You four-lane the highway to Whangarei instead of just going to Warkworth, you release an enormous amount of land. What does it do with that? Here's an answer, tell a hell of a lot of debt off its balance sheet."

Moving the port would also free up some of the best real estate in the city, Sir John says. 

"Who knows what you do with the land down by the waterfront, but what you do know is 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."

Helen Clark.