A dredging and disposal company has been given the green light to dump even more sludge into the sea off Great Barrier Island.
The Environmental Protection Authority (EPA) has granted Coastal Resources Limited a 35-year marine consent to dispose of 250,000 cubic metres of dredged sediment each year.
The company already has permission to dump 50,000 cubic metres of sediment at the disposal site - known as the Northern Disposal Area - which is 25 kilometres east of Great Barrier Island.
In June last year, it applied to expand its operation.
Coastal Resources Limited dredges material from sites around Auckland and Waikato, including marinas.
The EPA received 76 submissions about the application and the vast majority of them opposed the granting of consent.
Opponents included the Great Barrier Local Board and the Department of Conservation.
But in its final decision, the EPA's decision-making committee said that any potential adverse effects on the environment of any disposal would be restricted to the disposal area and negligible beyond the boundary.
Further, any potential adverse effects on existing interests, like commercial fishing, would also be negligible beyond the boundary.
Coastal Resources Limited will have to carry out regular environmental monitoring of the dumping site.
Any biosecurity risks will have to be notified to authorities within 48 hours.
Great Barrier Island Local Board chair Izzy Fordham said they were disappointed by the EPA's decision.
"We're no scientists by any sense of the word, our concerns were for the environmental impact of it, the effect on mammals and sea life, and the whole biosecurity risk that goes with it."
The local board does not have the resources to be able to appeal the decision, Ms Fordham said.
"I just sincerely hope that it doesn't impact on our ocean environment - in 2019 we really need to be doing better for our oceans."
Local iwi also opposed the consent application - as did the Auckland Conservation Board.
Its member and marine scientists Andrew Jeffs said Coastal Resources never addressed their concerns about the impact of vessel noise on marine mammals.
"It can stop them feeding, it can mess up social groupings, it can stop them from communicating with one another and create difficulties for them in finding food."
Dr Jeffs said it was surprising the EPA gave Coastal Resources the go-ahead, despite rigourous scientific evidence about the problems underwater sound can cause.
The Hauraki Gulf was recognised internationally as one of the most pristine habitats for marine mammals, he said.
While it was more expensive, Dr Jeffs said other options for dumping sediment on-land should be considered.
"Many of the people who own boats in marinas around Auckland are people who love the Hauraki Gulf, they love the fact that they see marine mammals when they're out on their boats and they'd probably be prepared to pay a bit more for their marina space in order to protect what they love about the Gulf."
Coastal Resources did not respond to requests for comment.