A study for Regional Council said, "Whaingaroa Harbour began to fill with sediment at least 8000 years before present (B.P.) and before the sea had reached its present level 6500 years B.P. Rapid sedimentation in the harbour before 6500 years B.P. is attributed to the formation of now relict intertidal shore platforms up to 700-m wide and ≤10 m below present-day mean high water level. These coastal landforms were rapidly formed 8000-6500 years B.P. by physical weathering of soft mudstone cliffs and wave action. Consequently, all but the upper two metres of the present day sediment column was deposited before 6000 years B.P. and thousands of years before the arrival of Maori some 700 years ago. Today, the harbour has largely infilled with catchment sediment up to ~8-m thick, with 70% of its high tide surface area being intertidal." It concluded that most sediment is now swept up to 20 km out to sea.