Cargo handled at the Port of Tauranga is expected to grow by up to 130 per cent over the next 30 years, if Tauranga continues to out-perform the Port of Auckland.
A report has spelt out the impact of Tauranga's greater competitiveness and efficiency, and the consequences if the conflict behind Auckland's waterfront dispute is not resolved.
However if Auckland is able to turn itself around then the outlook is that both ports will experience similar growth rates, the report said.
The long-term scenarios for growth were outlined in an independent 230-page report commissioned by local authorities in Northland, Auckland and Tauranga.
The $295,000 study by PricewaterhouseCoopers also warned of "increasing community sensitivity" to the increasing volume of cargo carried to the Port of Tauranga in trains - particularly along The Strand and the Otumoetai harbourfront.
Tauranga's cargo would grow by between 2.1 and 2.7 per cent a year, based on a comparison between the two ports prior to the waterfront strikes in Auckland that saw ships diverted to Tauranga. Auckland's growth was forecasted to be 2.2 to 3 per cent.
If the industrial action at Auckland became a long-term trend, the Port of Tauranga would surge ahead, with growth of 2.3 to 2.9 per cent a year, compared with Auckland's 1.6 to 2.5 per cent.
The other advantage of Tauranga, compared to the more confined docklands of Auckland, were the consequences of the Port of Auckland reaching capacity.
The report said the share of cargo carried through Tauranga and Auckland was likely to depend on each port's capacity to move additional containers. If Auckland reached capacity while Tauranga still had spare capacity, it was likely that that Auckland would have to raise its prices - encouraging shippers to divert.
Under the similar growth rate scenario, Auckland's container trade would grow by up to 2.5 per cent a year, compared with Tauranga's 2 per cent. If Tauranga held its competitive advantage, growth in containers would rise by up to 2.2 per cent a year, compared with Auckland's 1.7 per cent.
The biggest difference for Auckland in the competitiveness stakes was the bulk cargo trade. If Auckland could not match Tauranga, the annual growth in these cargoes for Auckland would slip nearly a third to 1.3 per cent.
The increase was less marked for Tauranga which already handled most of the bulk cargoes in the upper North Island.
Tauranga City Council growth funding and policy advisor Andrew Mead said said there was nothing in the report that the council did not already know. There would be issues with roading and rail as the port continued to grow.
He said transport infrastructure investment outside of the Bay of Plenty was also critical to enable the port to go forward.
Councillor Wayne Moultrie highlighted the long-term importance of the Sulphur Point container terminal extending southwards by up to 385m. Mayor Stuart Crosby said improved technologies would get around the problem of container cranes intruding into Tauranga Airport flight paths.
Mr Crosby said the report was mainly for Auckland, with Auckland picking up most of the costs.
He said shipping lines really controlled shipping in New Zealand and the ports were a "bus stop".