A 35-year-plan to dig up iron sand in Taranaki waters has been rejected by the Court of Appeal.
Trans-Tasman Resources hoped to suck up 50 million tonnes of iron sand each year, but lost its appeal against the High Court decision.
Greenpeace oceans campaigner Jess Desmond says it took a group effort to stop.
"This is a huge victory for the ocean and also for the local iwi, Kiwis Against Seabed Mining and for all the community and environmental groups who stood up to fight against this mine for years," Desmond told Newshub.
She said the waters are home to protected species.
"This is a clear message to the seabed miners, who are waiting in the wings, that there's too much at stake for risky, environmentally destructive operations to go ahead in New Zealand waters."
Forest & Bird is also celebrating the win. Chief executive Kevin Hague said the group won't stop fighting environmentally harmful proposals.
"New Zealand has a chance to build an economy that protects and restores the environment and doesn't profit from destroying it.
"This would likely kill everything on the seafloor, and severely disrupt the habitat of blue whales and other marine mammals."
Trans-Tasman Resource said on Friday the scheme would have created 300 jobs and added $160 million to the economy.
The company said the environmental effect of the operation would have been temporary and marine mammals, fish and sea birds would not have been affected.