Environmental groups are lodging a legal challenge against the permission given to mine for iron sands off the South Taranaki coast.

The Environmental Protection Agency earlier this month approved a second application by Trans Tasman Resources to extract 50 million tonnes per year - between 22 and 36 kilometres offshore.

On Thursday, Forest & Bird and Kiwis Against Seabed Mining announced they were filing appeals in the High Court.

"We think the EPA's decision to grant consent fails to protect the environment, and doesn't meet the requirements of the EEZ Act," F&B chief executive Kevin Hague said.

The area is home to more than 30 species of marine mammals, including some that are critically endangered, including blue whales and Maui's dolphins.

It was also a migratory corridor for humpback whales and there were little blue penguins present.

Mr Hague noted the EPA's decision-making committee was split and the chairman had to use his casting vote to decide the application.

"The committee members that would have declined mining consent believe that the environment is not protected in this decision."

Trans-Tasman Resources is planning to spend $600 million on the project.

It involves a slow-moving seabed crawler vacuuming up sand and seawater and pumping it to a vessel where the iron ore is magnetically separated.

The de-ored sand, about 90 per cent of the total, is immediately re-deposited.

TTR says it could generate 300 jobs in the Taranaki area and 1600 nationwide.

There were more than 13,000 submissions opposed to the project.