A report into an outbreak of gastroenteritis in Darfield says the drinking water supply was contaminated by animal effluent.
Twenty-nine peopletested positive for the bacteria campylobacter, with 109 likely to have contracted the illness.
The report by the Canterbury District Health Board's community and public health division, released on Tuesday, saysthe bacteria entered the water supply in July and August last year.
Medical Officer of Health Ramon Pink says heavy rain caused the effluent to enter a well that was not adequately treated.
Dr Pink says the report makes several recommendations to the Selwyn District Council, including a multi-barrier system to protect, monitor and maintain the drinking water supply.