Investigations are underway at Tāhunanui Back Beach in Nelson after contaminated material was found during routine sawdust testing.
The material is said to be sawdust from a local timber mill in the 1960s, and erosion left the material exposed over time.
Group manager community services Andrew White said a decision was made not to remove the material last July.
"We understood it was not considered a pollutant to the beach and inlet and would break down over time.
"We were also concerned that removing the sawdust would hasten the erosion occurring in the area."
However, White said his team took the approach to continue monitoring the erosion of the material and regularly testing it for contaminants.
"We tested the sawdust, and this testing has confirmed that some of the sawdust originated from pieces of timber that had been chemically treated with a preservative to prevent them from rotting."
The exposed substance is unlikely to be a threat to human health as the contaminant concentrations are below the human health protection standards for recreation land use.
However, arsenic, chromium, and copper levels are higher than the ANZECC sediment quality guidelines, which assess the potential risk associated with the material's potential for eroding into the marine environment.
This means the material could endanger the environment.
"Depending on the results of this investigation, it is quite possible that we will need to remove some or all of the filled area," White said.
While excavations occur, the raised car park at Back Beach will be closed from Wednesday, June 14, until Friday, June 16.
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