When Ports of Auckland chews off another section of the prized Waitemata Harbour, it expects a long, costly battle to prove its green credentials.
Ports infrastructure general manager Ben Chrystall said the proposed expansion would inevitably have environmental impacts, but their extent would not be known until detailed studies were done.
The Waitemata is home to sensitive populations of orca, bottlenose dolphin, Bryde's whales and fish stocks such as snapper and contains a plethora of popular recreational beaches.
Marine and coastal experts say a 9ha port extension is unlikely to significantly harm such treasures but the port company would need to prove this in detail to secure resource consent.
By reclaiming land and changing the shape of the harbour, port authorities must keep numerous environmental factors in mind - marine life, tidal currents, pollution from dredging and the stirring up of buried contaminants.
In the 1990s the ports infamously dumped inner-harbour dredgings in a popular fishing spot near the Noises Islands. Two decades on, its approach is very different. It will try to recycle any dredgings into fill which will make up the foundation for the new port.
Mr Chrystall said fill from a harbour crossing tunnel or a CBD rail loop could be made into "mudcrete" and used for land reclamation.
"It could be a win-win for Auckland because you might have an inexpensive home for that spoil and it might advance the pace of reclamation."
This method was used when the Rangitoto Channel was dredged, winning the port a Green Ribbon award in 2009 from the Environment Ministry.
The port has also proposed deepening the harbour for super container ships, but more dredging of the Rangitoto Channel is at least 10 years away.
Marine mammal expert Rochelle Constantine, at the University of Auckland, said there were no resident whales or dolphins in the port's proposed area of expansion, but bottlenose dolphins and orcas were known to swim near the site.
She said marine mammals could be affected by marine construction, so care would need to be taken when the port did noisy underwater work.
A new plot of land in the harbour could deflect currents in the Waitemata in different ways, leading to greater coastal erosion or the washing up of sediment on beaches.
Marine scientist Mark Costello said these factors were usually considered by engineers, and it was likely those designing a new port would take tidal disturbance into account.
Dr Costello also said if the tide was not allowed to flush out the harbour, algal blooms could grow around Auckland's waterfront.
The consent process for the port's Fergusson container terminal took four years and millions of dollars. The proposed upgrade is even larger.