People gathering shellfish have been advised to avoid the Canterbury coastline between Motunau Beach and Akaroa Harbour because of marine toxins.
Canterbury medical officer of health Dr Daniel Williams says routine tests on shellfish samples taken from Sumner Beach has shown levels of diarrhetic shellfish poisoning (DSP) toxins of up to 0.26 mg/kg.
He says this is above the Ministry for Primary Industries' safe limit of 0.16 mg/kg.
"Anyone eating shellfish from this area is potentially at risk of illness," he said.
"Mussels, oysters, tuatua, pipi, toheroa, cockles, scallops, catseyes, kina (sea urchin) and all other bivalve shellfish should not be eaten."
Dr Williams warned that cooking shellfish did not remove the toxins.
He said symptoms of diarrhetic shellfish poisoning typically appeared within half an hour and could last for 24 hours, and might include diarrhoea, vomiting, nausea and abdominal cramps.