In a split decision, the EPA has granted consent to Trans Tasman Resources, which wants to mine 50 million tonnes of seafloor sand in order to export about 5 million tonnes of ironsand a year, for the next 35 years.
About 13,000 people opposed the application by Trans Tasman. Its first application in 2014 was turned down by the EPA.
Kiwis Against Seabed Mining (KASM) chairman Phil McCabe said a "whopping" 99% of more than 13,733 submitters opposed the application, including iwi, the fishing industry, recreational fishers and coastal communities.
Local iwi Ngati Ruanui are understood to be considering an appeal, as were other other parties.
Mr McCabe said KASM had to take "the only responsible route" by appealing the decision, on behalf of the future of coastal people and the environment and sea life.
"The only logical next step is to challenge that decision on their behalf," he said in a statement yesterday.
There is a 15-day period for appeals, which can only challenge points of law, and the consents will not start until any appeals are resolved.
Mr McCabe said the decision was a "dangerous precedent" for New Zealand’s marine environment and the EPA was "acting prematurely", given seabed mining was an "untested activity".
EPA chief executive Allan Freeth said two committee members, including chairman Alick Shaw, voted in favour of the project and there was a "strong dissenting view" from the other two. As a result, Mr Shaw used his chairman’s casting vote in favour of granting the consents, BusinessDesk reported.
Mr Freeth underscored it was highly "complex and challenging" for the EPA and the fact there was a split decision reflected that complexity.
The central issue of the application was the sediment plume, he said.
The committee found the sediment plume would have significant adverse effects on benthic (seafloor) life for up to 3km, because of light reduction and direct effects such as smothering.
"The impact on benthic life within the mining site, while being expected to be a 100% loss in the short term, is expected to be temporary in the view of the majority committee decision.
"Conditions have been imposed to monitor this recovery, and take steps to ensure it occurs over the medium to long term," Mr Freeth said in a Wellington media briefing.
Trans Tasman’s first application was rejected in 2014 when a committee ruled the environmental impacts of the proposal were too difficult to gauge on the evidence available. The company went back to the drawing board and a second hearing was held between February and May this year, BusinessDesk said.
The project has sparked controversy, as those opposed argue it will change the physical, chemical and biological nature of the seawater and degrade the quality of the oceans as a whole.
Forest & Bird chief conservation adviser Kevin Hackwell said the proposed mining area was home to critically endangered blue whales, possibly one of only five known in the southern hemisphere outside the Antarctic.
"It’s also habitat for at least a further 33 species of marine mammals, including Hector’s and Maui dolphins, and an important migratory corridor for humpback whales," he said.
Seabed mining, and return of sediment to the seafloor, could cause "catastrophic damage" and affect seabirds, fish, and marine mammals.
"It’s completely irresponsible to put New Zealand’s only resident population of critically endangered blue whales in the firing line for Trans Tasman Resources to suck up the seabed and make a buck," Mr Hackwell said.
• Chatham Rock Phosphate, which separately wants to mine the Chatham Rise seabed for phosphate, welcomed the decision. It is working on a new application, after its first application was also declined by the EPA.
