Many people in the West Coast have polluted drinking water which in some cases contains fecal matter.
A report by the West Coast and Canterbury district health boards says none of the region's water supplies complies fully with drinking water standards.
Between 18,000 and 34,000 cases of gastro-intestinal disease are reported in the region each year, the report says.
West Coast health protection officer Christopher Bergin says water quality can be particularly bad in poorer rural areas.
Mr Bergan describes the situation as appalling.