Ironsand mining off Patea would produce a sediment plume stretching south down the coast past Wanganui, research commissioned by mining company Trans-Tasman Resources (TTR) has found.
The company is working toward consent to mine many square kilometres of seabed in the South Taranaki Bight for the iron-rich particles in the sand. It would work from offshore vessels and export the raw material to Asia. It intends to apply for marine consents this month, chief executive Tim Crossley said.
He has released a summary of the project and of research commissioned into its effects. The company now proposes using a remote controlled machine on the seafloor to mine the sand, rather than a suction dredge up above.
Using the crawling machine would be cheaper and easier and halve the environmental effect.
The summary says the effect of the plume would be minor by the time it reaches shore, given the bight's constant waves and sand movement.
The mining would kill any life on the seafloor - species such as tubeworms - but the sand would be quickly recolonised. The summary from TTR says the effects of mining on wave height and direction, sand movement and fishing will be minor. The operation would have an exclusion zone of 1km around it.
Mining would be done by a remote-controlled 12m-long crawling machine, with sand pumped to a processing ship on the surface. The sand would have iron particles removed magnetically. They would then be rinsed with freshwater made on board from seawater in a desalination process.
The rejected sand would be thickened with brine. Then it would be released from a pipe 4m above the seabed back into pits left by the mining.
The chairman of the opposition group Kiwis Against Seabed Mining, Phil McCabe, was pleased to finally get some information about effects from the company.
"They've been making claims of minor effects. We've been saying show us the information that backs up that statement."
After a quick look at the summary, Mr McCabe said effects on marine mammals could be major, because recent research showed the bight could be a blue whale feeding ground. He said the number of jobs created was smaller than predictions, and other ways of extracting the iron resource would give New Zealand greater gains.
He warned that the present proposal could be a precedent-setter for mining larger areas, closer to the shoreline and extending further up and down the coast. He would like a moratorium on the project, to allow more information to be gathered.
The consent application will be handled in a nine-month process by the new Environmental Protection Agency (EPA), with its board of inquiry appointed by Government. The opportunity to submit will probably be publicly notified in November.
Mr McCabe said his group would be at a disadvantage in the consent process.
"We'll have 20 working days to figure out what they've taken years to figure out. They've spent a lot of money and we don't have millions of dollars to do that work."
South Taranaki tribes Ngati Ruanui and Nga Rauru are also doubtful about the proposal.
"Until Nga Rauru has been through its own scientific and maatauranga research processes to be fully informed of what this means in our tribal domain we do not welcome mining and will maintain our stance until all the impacts have been fully considered," Nga Rauru kaiwhakahaere Esther Tinirau said.