A Tauranga iwi says it will take legal action, seek a Waitangi Tribunal hearing, or could even blockade the harbour if Port of Tauranga pushes ahead with plans to dredge the harbour and expand its operations.

Ngāi Te Rangi, in a letter seen by RNZ, says it would move to take action if its concerns were not addressed.

In the letter addressed to the board, Ngāi Te Rangi leader Paora Stanley says: "To reflect how important the harbour is to us, we will find a way to show you what happens when you can no longer get juggernaut access to enter the harbour at will."

The country's busiest port has applied to dredge 1.8 million cubic metres of the seabed, and extend its wharfs - which line both sides of the harbour between Mt Maunganui and Tauranga - by nearly 1km. The plans also include reclaiming 5 hectares of land at Sulphur Point.

The iwi fears the plan threatens kaimoana and the health of the harbour.

"To get you to understand the importance of land and the moana to us, we will begin the process of investigating the implications of the Waitangi Tribunal case against you and your business supporters."

The port is currently going through a consenting process before the Environment Court, but Stanley said he has been told the Environment Court was trying to move the hearing to October.

Port of Tauranga declined to comment on the letter.

Ngāi Te Rangi leader Paora Stanley says the iwi fears the plan threatens the health of the harbour. Photo: RNZ / Justine Murray.

Stanley says the health of the harbour had noticeably declined since the first dredging licence was issued in 2011.

"The Port of Tauranga is wanting to dredge an extra 1.8 million tonnes of seabed down the harbour. When they've done it in the past with 2 million tonnes, it's changed the trajectory of the harbour and the condition of the harbour," says Stanley.

"We traditionally gather kaimoana, and there's been a change in the amount since 2011. It's much more limited, if not fully gone in places. A lot of horse muscles which were prevalent around here are gone. In addition, all the traditional pipi beads are gone."

He wanted the Port of Tauranga to meet with iwi to discuss its concerns.

"I've been trying for the last six months roughly to negotiate with the port to have a win-win situation," Stanley said.

"At this moment, they have come up with nothing ... so let's take the gloves off and start scrapping."

Stanley says police and the harbourmaster would be given two days notice before any blockage took place.

RNZ