A discharge of untreated sewage into waters at Diamond Harbour has prompted a public health warning, with residents and visitors urged to stay out of the water and avoid gathering shellfish.

Environment Canterbury chair Deon Swiggs said the organisation was responding to a noticeable sewage plume entering the harbour.

The health warning applied to Diamond Harbour, Purau Bay, Church Bay and Charteris Bay. Recreational use of the water, including swimming, was not considered safe because of the risk to public health.

Swiggs said water contaminated by human waste could contain disease causing micro organisms such as viruses, bacteria and protozoa.

People were also advised to avoid eating shellfish from the affected areas. If fish were caught and eaten, the gut and liver should be removed and the fish washed thoroughly in clean water before cooking.

The public was urged to avoid contact with the water until the health warning was lifted.