Acres of shellfish dead in north Auckland estuary

Acres of shellfish have died in the past month in a north Auckland estuary smothered by mud.

That's according to an environmental group that's urging the council to force developers to limit the amount of sediment they discharge from building sites.

The Long Bay-Okura Great Park Society says the sediment has led to mass death of cocklebeds in the past month

"The death mortality area is very localised round the Okura Estuary and north of the Weiti development," the group's deputy convenor Peter Townsend says.

The estuary's plight heightened tempers at the Hauraki Gulf Marine Forum on Monday, with one attendee calling it "the most brazen case of violation of the most highly protected area you can get under NZ law".

Mr Townsend struggled to contain his emotion as he described the pollution of the estuary.

"The mullet no longer fly, the stingrays are few and far between," he said.

"The shellfish beds are decimated - soon, we believe the shore birds will follow as their food sources decline."

The Long Bay-Okura Great Park Society says the sediment discharge from building sites is the biggest threat to wildlife in the marine reserve.

Their sampling over two years found every sample had higher sediment concentrations than the average found in all streams monitored by the Auckland Council.

More than 60 percent of samples had more sediment than the maximum recorded in every Auckland stream monitored.

The developers' resource consents place no limits on how much sediment they can discharge.

"At the moment, they're allowed to discharge whatever they like - as long as they have the engineering in place as planned, even if it's ineffective," Mr Townsend said.

The colour of the estuary water is an indication of fresh sediment run-off.

In an estuary like this one it'd be normal to see a few dead cockles, but Newshub saw hundreds of thousands of them dead and dying, as evidenced by a smell of decay.

Test results by the Ministry of Primary Industries from the start of this month could not pinpoint the cause of the shellfish death.

However it noted that "it probably wasn't caused by parasites, but that their gills appeared to be affected by organic matter". "The results were consistent with animals dealing with a lot of environmental stress... including sediment."

The Hauraki Gulf Forum will now prepare recommendations for Auckland Council.