It's feared a break-away iwi will undermine the submission of other hapū against the dredging of Whangārei Harbour.
Refining NZ wants to make the channel deeper and straighter to give ships loaded with more oil access to the Marsden Point Oil Refinery.
Local iwi and hapū had been united in opposition to the move, arguing it will badly impact shellfish, marine mammals and the environment.
- Concern over plans to dredge Whangārei Harbour
However, in a letter to the Northland Regional Council (NRC) dated February 23 2018 the Ngātiwai Trust Board withdrew its opposition.
"Ngātiwai and Refining NZ have executed a private agreement, which addresses the concerns raised in Ngātiwai's [previous] submission," chief executive Kris MacDonald said in the letter.
Another Māori trust, Patuharakeke, has been coordinating the tangata whenua response to the resource consent application by Refining NZ.
Hearings are planned at Takahīwai Marae on Wednesday, and trustee Juliane Chetham said they had hoped to present a united front.
"[We're] surprised and disappointed that Ngātiwai Trust Board has decided to withdraw, particularly in light of the fact that they played a very major part in the cultural effects assessment," she told Newshub.
"At face value I think it does undermine [a unified hapū and iwi approach], but as the true mana whenua, mana moana, of that area - the people who have a Marae there, who are monitoring Mair Bank, who are out on the harbour all the time – it doesn't diminish our desire to protect this taonga."
When contacted by Newshub, Mr MacDonald said the private agreement was confidential, but had come after extensive consultation with Refining NZ in recent months.
When asked whether he thought the trust's decision reflected the feeling of the 2700 registered members of Ngātiwai, Mr Macdonald said the trustees had been voted in by the iwi to be their voice, and that they were entitled to make such decisions.
However, in November 2017, RNZ reported that 11 hapū had filed urgent claims with the Waitangi Tribunal about the Ngātiwai Trust Board's mandate to act on behalf of the iwi.
It follows a vote on the mandate in which fewer than 800 Ngātiwai members turned in a ballot, a participation rate of 28 percent.
Long term: How do the benefits compare to the drawbacks?
Refining NZ wants to move 3.7 million cubic metres of sand to make the Whangārei Harbour channel deeper and straighter.
The company has been working on the plan for the last four years and says it's the only way that the Marsden Point Oil Refinery can stay competitive.
Chief executive Sjoerd Post said ships can currently only enter the harbour under loaded which is not cost effective.
"The refinery in Whangārei is in competition with refineries in Korea and Singapore. Our customers have a choice - they already import 30 percent of their products, and they could easily one day tell us, 'look mate, we're going to import all of our products'," Mr Post told Newshub in the past.
"The experts actually think on balance, bringing these ships in fully laden - and therefore fewer ships - is actually safer than what we do today, so that could actually be a really positive outcome of what we propose."
Mr Post said dredging the harbour is also important for the local economy.
"This project is of real interest to the 500 Northlanders who come through the refinery gates every day, as well as their families, their community, and our business partners who help to keep the refinery running safely and reliably."
However, that's disputed by Juliane Chetham of Patuharakeke. She said there is no strong evidence that the wider region will see any economic benefits.
"The only thing they've been able to say is that this will prolong the life of the refinery, which in our view is a sunset industry anyway, but they can't even give figures or facts on how long it'll be able to prolong the life of the refinery.
"This will be a permanent change to our harbour for something that, as yet, we haven't seen a great deal of evidence for."
Ms Chetham says they're concerned about the effect maintenance dredging every two to five years will have on the pipi beds at Mair Bank, "which is already in a precarious state".
"If it could have been demonstrated to us that there were major positive economic benefits that would trickle down to our people and those that live in the area then sure we would consider it," she says.
"However, we haven't seen that, we haven't seen anything that would make us think it was worthwhile to take that risk."
Hearings were held on Monday and Tuesday by NRC at Toll Stadium, at which Refining NZ presented its case. The hearing at Takahīwai Marae on Wednesday will hear from tangata whenua. Two more hearings will be held at Toll Stadium on Thursday and Friday to hear from a variety of local groups and businesses, before a final hearing on March 13.