The exploration permit is just off the Taranaki coast near New Plymouth.

A mining exploration permit has been quietly granted inside a marine sanctuary off the Taranaki coast to protect the endangered Māui's dolphins.

The decision, which was approved in May, has shocked conservation groups who were unaware of it.

But the Department of Conservation (DOC) had been aware of Ironsands Offshore Mining Ltd's application since March. It has voiced "significant concerns" about the safety of the dolphins if the exploration proved successful and mining was to go ahead.

SIMON O'CONNOR/STUFF Debbie Ngarewa-Packer, of Ngati Ruanui, says she was shocked to hear of the permit being granted. The iwi is one of 11 organisations appeal Trans Tasman Resources consent to mine off the South Taranaki Coast. (File Photo).

In March DOC warned Conservation Minister Eugenie Sage that it was uncertain whether the public was aware of the application and there would be a "high level of interest" if people were told of the development.

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The exploration permit was later granted without seeking resource consent from the Taranaki Regional Council because its coastal plan rules exploration is a permitted activity.

SUPPLIED/TRANS-TASMAN RESOURCES The 66 square kilometres off the South Taranaki coast where Trans Tasman Resources has applied to mine iron ore.

Ironsands also has a permit to explore the seabed of Waihi Beach, in the Bay of Plenty, but this will require recourse consent from the regional council because its coastal plan requires it.

A Taranaki iwi already fighting mining plans off the coast of Pātea in South Taranaki said the approval raised alarm bells.

In an emailed statement, Te Runanga o Ngati Ruanui Trust kaiarataki Debbie Ngarewa-Packer said granting the five year exploration permit in the sanctuary, which runs from Oakura to Maunganui Bluff in Northland, set a dangerous precedent.

"Taranaki has been instructed by this coalition government to transition our economy away from fossil-fuelled industry. To do that we must have certainty that our unique appeal, including natural resources, will be protected," she said.

In April, Prime Minister Jacinda Ardern announced the end of offshore oil and gas exploration, about one month before Ironsands Offshore Mining's exploration permit was granted.

The permit covers an area almost four times the size of that granted by the Environmental Protection Authority to Trans Tasman Resources (TTR) for mining of the coast Pātea.

The EPA's decision was appealed by 11 parties and an appeal hearing was held in the High Court at Wellington in April. Justice Peter Churchman has yet to reveal his decision.

Ngati Ruanui was among the appellants and Ngarewa-Packer said she was concerned with the latest permit - although only for exploration - being bigger and closer to shore.

She warned the Taranaki coastline is at risk of becoming an ironsand mining wasteland instead of a progressive economy focusing on tourism, futuristic technology, innovative foods and a sustainable natural environment.

Greenpeace has also slammed the decision. Executive director Dr. Russel Norman said he was "gobsmacked" to hear exploration could occur in the sanctuary and has called on the Government to reject all attempts to mine the seabed.

In an emailed statement he said the area was also frequented by rare Southern Right Whales and close to the newly discovered New Zealand population of endangered Blue Whales.

Taranaki Regional Council director resource management Fred McLay said in an emailed statement the permit process did not involve the TRC.

"Until the council views a copy of the decision it cannot comment on the proposal. However, under our current Regional Coastal Plan for Taranaki and Proposed Coastal Plan, minor seabed disturbance activities associated with exploration are permitted (i.e. no consent required if the environmental conditions are met)."

On a visit to New Plymouth yesterday, Acting Prime Minister Winston Peters said he was not aware of the permit and was unwilling to comment about it until he had that information.

"It's possibly a case where this is not the end of it."

New Plymouth District Mayor Neil Holdom declined to comment until he knew more about it. "I've obviously got some homework to do," he said.