Forest & Bird northern regional manager Nick Beveridge claimed the dredged material could impact on corals, shellfish and marine life on the sea floor.
In a report provided to the EPA, the Department of Conservation (DoC) said the area included a ranged of protected species, including whale sharks, manta rays, leatherback turtles and spine-tailed devil rays.
"The Department is concerned with the lack of reliable information on the location of
threatened species and rare and vulnerable ecosystems in the vicinity of the proposed
disposal site," the report said.
There was also a "significant risk" the sediment could smother threatened marine organisms, DoC said.
Beveridge said they were also concerned about potentially contaminated materials being dumped in the area, calling it "contaminated sludge".
They wanted POAL, which was owned by Auckland Council, to dump the material in landfills instead.
Ball said it was POAL's "firm view" Forest & Bird's concerns were "unfounded".
Conditions set by the EPA in granting the consent ensured very high sampling, testing and quality standards had to be met before any material could be disposed of at the site.
Any material found to be contaminated would be disposed of in landfill.
Most of the dredging would take place in the Rangitoto Channel, where the material was uncontaminated sea bed, Ball said.
"Filling Auckland's landfills with mudstone from the channel would be a terrible waste of scarce landfill capacity."
Previously dredged material had been used to create new port land, but given the commitment to end reclamation they needed a new area to dispose of the material.
The "only viable alternative" was disposal at sea, Ball said.
Forest & Bird had been briefed about the work back in May, and all technical reports made available to them, Ball said.
The authorised offshore dump site was the closest one to Auckland, was 700 to 1200m deep and 15km wide.
The EPA decision concluded while the potential impacts within the dump site - which had been used historically for similar dumping and even "unexploded ordnance" - could be significant, but outside the site would be "at worst, minor".
EPA conditions included a maximum of 2m cubic metres of capital dredgings over the 35 years and 50,000 cubic metres of maintenance dredgings annually, that material needed to be tested to ensure it did not breach contamination limits, monthly reporting published online, and checks for marine mammals took place before any dumping.
In February, the EPA granted consent to Coastal Resources Limited to dump 250,000 cubic metres of sediment a year for the next 35 years off Great Barrier Island.
The sediment would come from Auckland City infrastructure spoils, marinas in Auckland and Waikato, Ports of Auckland dredgings and a proposed America's Cup village at Hobsonville Pt.
The decision has been appealed by Ngati Rehua Ngatiwai ki Aotea and The Society for the Protection of Aotea Community and Ecology.
