An application to mine millions of tonnes of iron sand off South Taranaki has been approved by the Environmental Protection Authority (EPA) in a split decision.
Environmental group Kiwis Against Seabed Mining, which said it was "stunned" the project received the green light, said it would appeal the decision.
Trans-Tasman Resources had asked to mine for iron ore in a 66 square kilometre area of seabed off the Taranaki Bight, the first ever proposal for that type of project.
The company intended to dig up to 50 million tonnes of iron sand each year, extracting the iron ore and dumping the residue on the sea floor.
It said the scheme would create 300 jobs and add $160 million to New Zealand's GDP.
The authority granted the application in a public announcement in Wellington this morning.
EPA chief executive Allan Freeth said it was a split decision, with decision-making committee chairman Alick Shaw issuing a casting vote in favour.
Kiwis Against Seabed Mining chairman Phil McCabe said appealing the decision was "the only responsible route".
BREAKING: we’re stunned at EPA green light to seabed mining today. We will appeal decision, on behalf of all who love our oceans.— KASM (@KASM_NZ) August 9, 2017
"We are stunned that the EPA could have given this experimental industry the go-ahead, given the startling lack of available crucial information," he said.
"Even the EPA's decision-making committee was split on its decision."
Nearly 14,000 people had filed submissions opposing the application, Mr McCabe said.
The decision set a dangerous precedent, he said.
"There are further proposals closer to the coast in the South Taranaki Bight, off the coast off Kawhia, the South Island's West Coast, the Chatham Rise and an area off the coast of Waihi Beach."
Trans-Tasman Resources' first application in 2014 was turned down by the authority, which found there was uncertainty about the potential of effects on the environment and on existing fishing and iwi interests.
During a four-month hearing that ended in May, Greenpeace and Kiwis Against Seabed Mining said the authority should say no again.
The extra information backing the proposal was still too sketchy, the groups said.
The groups won a court case late last year demanding the release of further documents that Trans-Tasman Resources had redacted, saying they were commercially sensitive.
Trans-Tasman Resources said during the hearing that dredging and backfilling of sand would have a negligible effect, compared to passing ships or from oil and gas-drilling.
The mining would not take place in a pristine environment as the area already had a lot of commercial activity, the company argued.
"We go straight to appeal'
A South Taranaki iwi said it would appeal the decision to approve the application.
Ngāti Ruanui kaiarataki Debbie Ngarewa-Packer watched a livestream of the EPA decision, while surrounded by supporters at the Patea Area School.
Ms Ngarewa-Packer said she was disappointed but not surprised by the decision and the fight would go on.
"We go straight into appeal. So our lawyers and our team have been prepared and ready and it will be based on the same issues we had all the way through, with the redacted areas, with the poor process, the way they dropped the ball the whole way through."
A condition of the application's approval was that Trans-Tasman Resources work with iwi, but Ms Ngarew-Packer said Ngāti Ruanui would politely refuse to cooperate.