Auckland Council guidelines intended to prevent sediment runoff from housing developments into waterways can't cope with heavy rainfall, according to an independent marine consultant.
In a report commissioned by the Long Bay Okura Great Park Society, Ecoast also claimed Auckland Council wasn't doing enough environmental monitoring of sensitive marine sites.
ECoast Managing Director Dr Shaw Mead called the guidelines a "tick the box exercise".
"They're just not up to heavy rainfall in a sensitive environment."
Once the rainfall exceeds 15mm within a 24-hour period, the efficiency of the council's sediment control guidelines drops to below 60 per cent, Dr Mead says.
But Weiti Bay developer Evan Williams disagrees.
"We are confident that the sediment management systems we have invested in will continue to be effective during construction.
"Water treatment systems, vegetation of slopes and the lining of gullies with coconut matting will ensure minimal construction sediment will enter the Marine Reserve from Weiti," Mr Williams says.
Mr Williams argues that the report's data is not presented in a balanced fashion.
"There is no justification for the conclusion reached in the report that the issue is construction sediment.
"The reverse is the case."
He says it's problematic that Dr Mead's report is based on data collected by the Long Bay Okura Great Park Society, because the group has a long history of opposing development and is not independent.
Dr Mead says the Okura Estuary has high ecological value - it's one of the most important small estuaries on the East Coast.
It's also in a marine reserve and it's already under sediment stress, meaning it needs special protection.
Dr Mead says there's no single answer to fixing the problem, it could require changes to urban planning, engineering methods and policies, so he wants a working group set up to address it.
Although Dr Mead was commissioned to assess the impact of Weiti Development on the estuary, he says other land users also contribute to the sediment runoff.
"There are multiple developments in the catchment, and it's even down to single house lots.
"That's just the beginning of the Weiti Development, it's all stabilised now and covered with grass but every one of those sections will be ripped open again to build each house.
"It's death by a thousand cuts, those little bits that keep going in and swamping the environment."
Auckland Council's chamber was packed with members of the community as Dr Mead presented his findings to the environment and community committee.
Long Bay Okura Great Park Society spokeswoman Fiona McLaughlin says the marine reserve is supposed to have special protection, to provide a home for endangered species and other wildlife.
"Instead, this precious marine reserve is becoming a place of death, and it will only get worse if the Auckland Council doesn't take urgent action," she says.
Mr Williams of Weiti Development says he wants to work with the groups interested in protecting the marine and land environment, but Long Bay Okura Great Park Society's Peter Townend refuses to speak to him.
The committee will refer the findings to council staff and has requested a report with options for better sediment management, due next month.
The council has devices currently monitoring the quality of the Karepiro Bay and the streams that feed into it, which will provide the data for a whole-of-catchment study due to be released later in the year.
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