Auckland developer claims he's not to blame for acres of dead cockles

The mystery surrounding the deaths of thousands of shellfish on Auckland's North Shore deepens, with a developer hitting back at claims his construction is to blame.

An environmental group says the cockles were smothered by sediment from building sites, but the director of Weiti Development says he's spent millions to reduce his developments' environmental impact.

The sprawling piece of land overlooking Karepiro Bay on Auckland's North Shore used to be pine forest, but soon multi-million dollar houses will appear in its place.

Developer Evan Williams says it'll still be an improvement to the landscape.

"My vision has always been for a huge conservation estate with very little development in it," he said.

Environment groups claim sediment run-off from Weiti Development contributed to the mass death of cockles in the Long Bay-Okura Marine Reserve in March.

"What's being said is not true, and I do care," Mr Williams said.

He says he's spent about $15 million on measures to prevent run-off - including ponds to filter the sediment, planting trees, and covering the hillsides with a coconut matting to prevent freshly sown grass and soil washing away when it rains.

But he accepts that sometimes his systems fail.

"There are occasions like these three recent cyclones where we've been getting up to 90mm of rain in a day, and that will overtop anyone's system," Mr Williams said.

But he says most of the sediment that washes down this stream is from other land uses in the 8000-hectare Okura catchment, such as forestry and farming.

In a statement the Auckland Council said compliance at Weiti has been consistently high, and monitoring of the Okura Estuary over the past 18 years shows on-going declines in ecology, and increases in sediment which can reduce the resilience of shellfish.

A council study into how land use changes affect the marine reserve is underway.  Its results are expected later in 2018.

Mr Williams supports it, and says it'll help everyone understand how to better protect the area.