Refining NZ wants to deepen the channel into Whangarei Harbour so tankers can bring in larger shipments of crude oil.

Local hapu, concerned about kaimoana beds and other harbour issues, are not sure it's a good idea.

Refining NZ is ready to front the public about its plans to deepen and straighten, a $57 million proposal it first aired in 2014/15.

This second stage is being revealed after more than two years of discussions with tangata whenua, local residents and other interested parties.

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Some parties with a stake in the harbour and who have already been involved in consultation for several years are not ready to give the project the thumbs up.

"The refinery has definitely front-footed the consultation process and we and other hapu around the harbour have been involved in meetings and consultations along the way," Patuharakeke Trust secretary and environmental portfolio holder Juliane Chetham said.

The trust is writing its own cultural impact assessment, planning further consultation and also awaiting Refining NZ's final cultural and scientific reports.

"Until we've worked through that process, we haven't a got a definitive opinion but the majority of tangata whenua are already concerned about the state of the harbour and resources such as the Mair Bank pipi beds," Ms Chetham said.

Following the Rena disaster in Bay of Plenty, Northland was identified as the region most at risk of a major oil disaster because of the refinery site.

There have been incidents in the past where fully laden oil tankers hit the bottom of the harbour at Marsden Point.

Those incidents highlighted problems with the channel at that time.

They also drew attention to the vulnerability of the coastline because of the position of the refinery and the constraints of that particular channel into a deep water port.

In 2004, the then Whangarei MP Phil Heatley raised the matter in Parliament.

To enable the existing channel to take bigger or heavier ships, 3.7 million cubic metres of sand would have to be moved, mainly from outside the harbour.

Refining NZ said dredging would involve mitigation work to protect Mair Bank and other fragile areas in Bream Bay.

The company wants the channel deepened so bigger tankers can come in fully laden, reducing shipping movements and transport costs.

Those big ships already come in, but carrying less than capacity loads because the channel is not deep enough.

The economic argument includes allowing the refinery to keep pace with imports from competitive Asian "mega-refineries" - one reason Northland MP Winston Peters has described the proposal as a "no-brainer" and good for Northland's future.

Chief executive Sjoerd Post said the first round of consultation in March 2015 proved an important test of the proposal with different groups.

It brought in a lot of feedback about the location for dredgings, relief that much of dredging would be outside of the harbour, the importance of Mair Bank and the role of the refinery as a major employer, Mr Post said.

"The other clear message to the company and the independent experts was that the science behind their studies needed to be solid and that their findings should 'tell-it-like-it-is'."

Environmental groups and recreational harbour users, such as fishers and boaties in either clubs or as individuals, are expected to be among stakeholders making submissions.

The proposal, dubbed 'The Deeper Story', will be explained at a pop-up container/kiosk at various Whangarei locations, starting on Saturday and Sunday in the Whangarei Town Basin, or on the website www.deeperstory.co.nz