Consent to mine ironsand off the south Taranaki coast was approved without proper data to show how many endangered species live there, the High Court has been told.
The second day of submissions on an appeal against the consent is under way in Wellington.
The consent allows up to 50 million tonnes of ironsand to be dug up from the ocean floor each year.
Five million of that is iron ore but the rest will be dumped back on the bottom of the sea.
Royal Forest and Bird Protection Society lawyer Martin Smith said at least seven endangered species including Hectors and Māui's Dolphins, were know to frequent the area, but there was a lack of data around this.
"The information may well be the best available... but the best available may still be inadequate to make a decision."
He said the Environmental Protection Authority (EPA) could have declined the application from Trans-Tasman Resources and favoured caution and environmental protection instead.
Consent for Trans-Tasman Resources to mine the area was granted by a decision-making committee in August last year.
That decision was opposed, and is being appealed this week.
Another group appealing the decision was the Taranaki-Whanganui Conservation Board.
Its lawyer James Gardner-Hopkins said the Authority should have given more weight to other marine laws, as is required under the Environmental Effects Act.
Mr Gardner-Hopkins told the court the New Zealand Coastal Policy Statement (NZCPS) should have been considered, as it says the dumping of sediment is to be avoided.
"When the EPA is taking into account the RMA [Resource Management Act] and the NZCPS as it is required to do - when it looks at the language used in the NZCPS about avoiding increases in sedimentation and discolouration and the like, that is a very strong directive," he said.
"The decision making committee just did not give that genuine thought and attention."
Today Greenpeace and Kiwi's Against Seabed Mining started their submission.
It is expected Trans-Tasman Resources will give theirs within the next two days.