Auckland has a choice if it wants to open up more of its waterfront - let the port expand into the Waitemata Harbour or lose cargo to Tauranga and Northland.
An independent study on freight demands of the three North Island ports shows Auckland cannot open up Captain Cook and Marsden wharves to the public without having to fill in more of the harbour or limit the amount of cargo it handles.
That would lead to the economic benefits from Ports of Auckland going to Port of Tauranga and Northport.
The study was sparked by a Herald campaign against further reclamation of the Waitemata Harbour. That idea caused a huge public backlash and a rethink by the Auckland Council.
Ports of Auckland wanted a coastal zone allowing it to expand its waterfront operations from 77ha to 95ha by 2055 - the equivalent of adding two Victoria Parks to its space.
The study also followed a Herald poll of 1000 Aucklanders in September, which found more than three-quarters wanted more wharf space opened up to the public.
The technical study, a 228-page document by PwC, said the Auckland port could cater for growth in container cargo through efficiencies but handling the growth in bulk storage was more difficult.
Losing the 3ha of bulk storage at Captain Cook and Marsden wharves would make matters worse.
Even with "very significant operational efficiencies", the study said, Ports of Auckland would still need extra berth and storage space by 2041 to cater for growth.
This would most likely involve further reclamations, but not as substantial as the extra 18ha envisaged in a 2008 planning document.
Heart of the City chief executive Alex Swney said the study was thorough and informative, but "bold it is not".
"Nowhere in this study can we see the value of an enhanced harbour and greater tourism appeal," he said.
"We need efficient port infrastructure, but we should not be lumping the next generation with a more industrialised-looking Waitemata port."
A Ports of Auckland spokesman said the study was in line with the company's view that the port was an important part of Auckland and the nation's infrastructure, it had to become more efficient and would need some "modest expansion" to meet future demand.
"If Captain Cook and Marsden wharves are released for public access, the capacity will need to be replaced or the port will lose business. Replacement capacity could come in the form of additional reclamation and by providing alternative facilities, like car-parking buildings.
"Losing business to other ports would reduce earnings and dividends to ratepayers, and would have an environmental cost, for example in the transportation of cars from Tauranga or Whangarei to Auckland."
Mayor Len Brown said the study would help in making informed decisions on the port.
He said the findings would allow for growth of the port, but "not right across to Devonport".
The study said building a new container port would involve huge financial and environmental costs which would outweigh the benefits.