Environment Canterbury is monitoring the region's Ashley River, after toxic algae was found in the water.

Dark brown and black looking mats in the water have been identified.

Canterbury Medical Officer of Health Alistair Humphrey says anyone feeling ill after coming into contact with the water should contact a doctor.

"They can get this peculiar numb feeling around their lips, they can develop rashes.

"Sometimes they can get stomach cramps if they ingest it."