A 'concrete commitment' by Northland MP Winston Peters to move Auckland's container terminal and car import trade to Northport has been praised and panned, but the NZ First leader says it makes the best economic sense for Northland, and New Zealand.

Mr Peters wants to move Auckland's car import trade to Northport at Marsden Point, within two years, and the container trade in a decade. He also wants to create special economic areas (SEAs) with tax free status around Northport, and later Southport in Bluff. Both moves are designed to spark Northland's ''economic renaissance''.

The plan has been panned by the Government and road transport leaders, but hailed by Auckland lobby group Stop Stealing Our Harbour.

Mr Peter said Aucklanders want their harbour back while Northlanders want the jobs and opportunity that would come from Northport's transformation. It's a cast iron commitment, he said, but conditional on his party being in key position after the election.

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"The days of the Ports of Auckland as a container port and as a car yard are numbered," he said.

"This will be matched by investment in rail freight and improved road links between Whangarei and Auckland (and a rail link to Northport). The benefits for Aucklanders of getting 77ha of their waterfront back in the 2020s is incalculable. For Northland, the port's expansion would spark Northland's economic renaissance with an unprecedented innovation for exporting.

"This (SEA) will be duty-free, GST-free and tax-free, opening the door to value-add manufacturing, logistics and re-exporting."

Northport said it is well positioned to support economic growth in Northland and Auckland.

"Along with a natural deep-water channel perhaps our most significant advantage is that we have more space at our disposal than any other port in New Zealand," the statement said.

"Significant growth is possible here . . . Northport is ideally positioned to handle substantial growth in imports and exports. We are ready to respond to whatever role we might be required to play in the development of Northland's economy, as well as Auckland's."

Whangarei Mayor Sheryl Mai wasn't sure how the SEA would work, but anything that brought economic activity to Whangarei was worth looking at.

She said moving Auckland's port activities to Northport had been talked about for a long time and she was not sure if Northport could handle all of Auckland's freight. "Northport already has a container crane and is already preparing for expansion. It is gearing up for this sort of thing and with fast-tracking the rail and roads, bring it on."

National leader Bill English called the idea a "red herring". But Labour leader Jacinda Ardern has committed to an evidence-based consideration of alternatives that are good for all of New Zealand.

"I've long held that any of the suggestions around the Ports of Auckland expanding at its current site ... really isn't something that we should be supporting, instead, if expansion's what's required, we should be looking at a range of other options," she said.

Stop Stealing Our Harbour welcomed the "fantastic news that could save our harbour from Ports of Auckland's relentless expansion and help transform Auckland into one of the great waterfront cities of the world."

Road Transport Forum Chief Executive Ken Shirley and CEO of National Road Carriers, David Aitken both criticised the plan.

Mr Shirley said forcing containers and cars from the Ports of Auckland to Northport is economic vandalism at its very worst and that would seriously damage New Zealand's economy. Mr Aitken said even after a massive upgrade of the rail from Auckland to Marsden Pt it could not handle the volume of freight now coming through the Port of Auckland

But Mr Peters said the two roading spokesmen had a mandate to promote and support road transport, not the wider issue of what was best for the whole country.

He said a new port in the Firth of Thames would cost up to $5.5 billion. Mr Peters said his plan was in the best long term economic interests of the whole country, and moving the freight to Northport would cost far less than the $5.5b to move it to the Firth of Thames.