The Canterbury District Health Board is investigating the source of gastroenteritis outbreak in the Selwyn District.
Nineteen cases have been confirmed in the townships of Darfield and Hororata, and more are expected to be suffering from the illness.
The DHB understands the outbreak may have come from contaminated water supplies.
Canterbury Medical Officer of Health, Dr Alistair Humphrey says they're carrying out both clinical and public health investigations, to determine the source of the outbreak.
Dr Humphrey says the drinking water is safe again for residents in these areas, however, water storage tanks on private properties may still contain contaminated water.
He says the safest approach is to drain the tanks, or use bleach in very large tanks to remove potential contamination.