Beautiful Mahurangi Harbour is slowly dying, filling with mud and silt that is suffocating marine life and threatening to create boating no-go zones.
After almost a decade of scientific study, the state of the harbour shows the diversity of its marine life declining with some areas now devoid of shellfish such as cockles.
"If this continues, in a few years the harbour will lose its water clarity, shellfish will die, oyster farming become less viable and fishing and boating will be affected," said Auckland Regional Council land and water quality manager Eddie Grogan.
The harbour is a marine playground for Aucklanders, lying just south of Warkworth and includes popular holiday spots.
The upper reaches of the harbour, including Hamilton's Landing and Te Kapa River, are in a worse state than the shallower reaches of Manukau Harbour, said ARC environmental scientist Dominic McCarthy.
"We've been monitoring the Manukau for 15 years and it doesn't show the same problems we are seeing in the Mahurangi," he said.
Though sediment first began washing into Mahurangi harbour around 150 years ago after deforestation, land clearing, subdivision and farming mean more soil washes into the harbour every year.
Core sampling shows between 4mm and 7mm washes into the harbour each year compared with less than one millimetre 150 years ago.
Mahurangi was more likely than other parts of Auckland to be hit by intense storms, Mr McCarthy said.
The shallow estuaries in the upper reaches of the harbour were becoming more shallow, and deeper parts of the harbour were also showing signs of stress. Horse mussels were struggling to filter ever-increasing amounts of silt.
ARC presented its findings to Rodney District Council, which already has some land-use rules in place, including designated areas deemed unfit for subdivision.
But ARC is putting $600,000 into a draft action plan to be agreed on with residents and the district council. It should be ready by July, and a dedicated coastal adviser will be appointed.
"Essentially Mahurangi is a bit of a pilot where we can put sediment controls into action, but we want the community to agree and be part of it," Mr McCarthy said.
Controls include planting around waterways to stop banks crumbling, keeping stock out of streams and rivers, tighter controls on earthworks and providing incentives to stop development in erosion-prone areas.
But Mr McCarthy warned damage to some parts of the harbour might be irreversible.
"If you asked if some places in the harbour would recover completely, I would say probably not - but if you asked if we could improve them, I would say yes," he said.