The complex and chaotic seas of the South Taranaki Bight should not be subjected to the risk of seabed mining, Ngā Rauru people told an Environmental Protection Authority committee on Monday.

Ngāti Ruanui was the first tribe to speak on the first day of the authority's seabed mining hearings in New Plymouth. It's the closest iwi to Patea, where mining is proposed offshore. At least 72 Ngāti Ruanui people were expected to attend.

The authority is considering whether Trans-Tasman Resources (TTR) can mine 65.7km of seabed 22-36km offshore from Patea, for up to 35 years.

After Ngāti Ruanui came Ngā Rauru. Its chairman Te Pahunga Marty Davis has been told the authority is legally obliged to apply the "precautionary principle" especially strongly.

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"The uncertainty and insufficiency of field testing is extremely concerning for an operation that involves 65 square kilometres of what is the equivalent of underwater open-cast mining," he said.

Approving the application would open the way for mining applications along the black sand coasts of the North Island, he said.

The iwi has had advice from legal experts from Victoria, Auckland and Otago universities, as well as writing its own submission.

On Tuesday the authority will hear from Whanganui iwi fisheries representatives, governance entity Ngā Tangata Tiaki and the Ngā Ruahine iwi. Also speaking will be Hawera's Karen Pratt, Taranaki Regional Council, Ngā Wairiki/Ngāti Apa, Tanea Tangaroa, John Milnes and Nicola Patrick.

The hearings are expected to finish in New Plymouth on Thursday, and resume in Wellington on March 15.

TTR's application to mine the seabed received 13,500 submissions, most of them opposed.

It's supported by the Ministry of Business, Innovation and Employment. Fairfax has reported that James Stevenson-Wallace, the general manager of its energy and resources division, told the authority the iron-sand was a "world class resource" which the ministry wanted to see developed.

"We believe that granting TTR's marine consent sends a positive signal to investors that will support New Zealand's efforts to support foreign investment into New Zealand," he said.

Developing New Zealand's mineral resources would be a critical component in helping to achieve targets set by the Government's Business Growth Agenda, he told Fairfax.