The High Court has overturned an application to mine ironsands from the South Taranaki seabed.
Trans Tasman Resources was initially granted a consent by the Environmental Protection Agency (EPA) to dig up 50 million tonnes of the South Taranaki Bight Seabed over 35 years.
That would allow them to mine 5000 tonnes of iron ore per year.
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Several environmental groups, fishing groups and iwi appealed the decision, including Greenpeace, Kiwis Against Seabed Mining, Ngati Ruanui and Nga Rauru.
That appeal was upheld on Tuesday, with Justice Peter Churchman ruling that the consent's "adaptive management" approach was inconsistent with New Zealand law.
Kiwis Against Seabed Mining described adaptive management as a process of "trying it out and seeing what happens, and adapting the conditions accordingly".
Appellants opposed the application because they said undersea mining would stir up sediment which would stay suspended in the water, degrading the living environment for fish.
Trans Tasman Resources lost its first application to mine the seabed but resubmitted to the EPA. The company claimed the mining scheme would create 300 jobs and add $160 million to the country's GDP.
Justice Churchman has sent the decision back to the EPA for reconsideration.
Greenpeace NZ executive director says the result is a "huge win for the oceans and for people power".
"Oceans are the life support system of our planet," he said in a statement. "They regulate the climate and provide the food and oxygen we need to survive.
"They contain and sustain most of the life on Earth, and they are already in crisis.
"Seabed mining would further threaten their ability to sustain life, including our own. I certainly hope this will be the last we'll see of these wannabe miners."