Northland Health is warning the public not to take shellfish from the Kaipara Harbour or along the coastline north to Maunganui Bluff.
Tests have shown the presence of Paralytic Shellfish Poisoning toxins - above the safety level.
The Medical Officer of Health, Dr Clair Mills, says eating kina, mussels, pipi, tuatua, oysters, cockles or scallops from the affected area could cause illness, and cooking does not remove the risk.
Dr Mills says toxin levels are also rising to the southof Kaipara harbour, and the warning area may be extended.
Symptoms of paralytic shellfish poisoning include numbness and tingling around the mouth or extremities; difficulty in swallowing or breathing; dizziness and double vision.
Dr Mills says anyone experiencing those symptoms should call 111.
