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A $600 million project to install giant tidal turbines in the mouth of the Kaipara Harbour is facing a wave of opposition.

Three appeals against the scheme have been lodged with the Environment Court - including one from the Conservation Department and another from northern Kaipara hapu, Te Uri O Hau.

Locals fear the Crest Energy proposal uses untried technology without knowing the full environmental effects.

The Northland Regional Council last month granted resource consent for a staged development, of 200 22m underwater turbines anchored to the seafloor, generating enough electricity to power 250,000 homes.

The company will also bury two 7km transmission cables from the turbines to Pouto Point on the harbour's North Head.

Crest Energy director Anthony Hopkins acknowledged the technology was experimental but said there was no evidence of anything other than minor environment effects.

"Nobody has an issue with generators," he said. "But for some reason when you put a slow-moving fan under water, people are worried about the technology."

The application attracted more than 240 submissions, almost evenly split between opponents and supporters. Opponents were concerned about effects on fishing, navigation and anchorage and marine life and tidal patterns. Supporters said the project would help New Zealand meet its Kyoto Protocol obligations and reduce fossil fuel use.

Tidal turbines follow similar principles to wind turbines but because sea water is 830 times denser than air, the same flow generates several hundred times more power in water than in air.

Crest said the project could contribute 3 per cent of New Zealand's electricity supply. It was awarded a $1.85m grant from the Government's Marine Energy Deployment Fund to develop the first stage.

The harbour, one of the world's largest, covers 900sq km with 3000km of shoreline. Eight billion cubic metres of water pass in and out of the harbour each day but so do vast amounts of sand.

Dargaville diver and shipwreck hunter Noel Hilliam said up to a million tonnes of sand entered the harbour annually.

"With any obstruction in this water, that sand is going to drop somewhere and they could end up with a sandbank," said Hilliam. "It will push the tidal flow out to either side."

He also said Crest had underestimated the difficulty of putting divers down in the harbour entrance, with powerful rips and zero visibility.

But Hopkins said this was no different to conditions experienced by the offshore oil and gas industry in other parts of the world.

"I don't think anybody would be glib about what nature can do. But the current there is really good."

Hopkins was confident Crest could address all issues and install the first turbines within 18 months. It still needs the Conservation Minister's approval.