Auckland ratepayers could face hefty hikes if the port leaves the city, Mayor Phil Goff has warned.
NZ First leader Winston Peters has reignited the debate over the port, which occupies prime real estate in the city's CBD, by calling for its relocation to Whangarei.
"Aucklanders want their harbour back while Northlanders want the jobs and opportunity that would come from Northport's transformation," he told NZME.
While Mr Goff backs opening up the waterfront, it's not as simple as just kicking Ports of Auckland out.
"There's got to be a business case. If you're going to move the port anywhere, does it stack up economically and does it work environmentally?" he told RadioLIVE's Mark Sainsbury on Thursday.
"Before you can say 'we're definitely going to do this', you need to answer these questions."
One of the questions is how Auckland Council, which owns the company, would make up the lost revenue. The port currently brings in around $60 million a year.
"That's the equivalent of about a 4 percent rate increase," said Mr Goff, who doubted neither National nor Labour would bow to Mr Peters' demands.
"No central Government is going to unilaterally say 'we're going to take away your asset and put it somewhere else'. That's not the way a democracy works. There's got to be a process of negotiation."
He said research shows neither Marsden Pt at Whangarei nor Tauranga could handle the amount of freight that passes through Auckland.
"If you were looking at everything going to another particular area, the work done so far suggests that's not possible."
But what could be done is splitting the load up more evenly.
"My thought has pretty much always been that we still need a port of Auckland. This is a city that's got 38 percent of the country's GDP. It doesn't make sense to shift everything in and out of either Northport or Tauranga... but if you want to make a case of Northport doing some of it, let's take a look at the business case, let's look at how you're going to compensate Auckland if you're going to take away a form of Auckland's income."
Mr Peters says Whangarei could work if the Auckland-Northland rail line is upgraded to handle the loads.
Labour leader Jacinda Ardern isn't opposed to moving the port, as long as it's not just to satisfy "particular interests around regional economic development" - perhaps a jab at Mr Peters, who is MP for Northland.
"You look out and see this car park that exists on our waterfront and think there's got to be a better way than this," she told Newstalk ZB on Thursday morning.
"Our view is that we've got to make a decision that's in the best interest of all of New Zealand."