Health officials are asking residents of Akaroa on Banks Peninsula to seek help if they feel unwell, after dead animals were found in the town's water supply.
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A boil water notice was issued on Friday and is expected to be in place for a few weeks.
Canterbury Medical Officer of Health Dr Cheryl Brunton said while the risk to people's health was low due to the large volume of water in the reservoir, people could still develop gastric illnesses.
She said chlorine in the water supply killed bacteria, but the possum and bird carcasses may have introduced other contaminants that could only be removed by boiling water.
Christchurch City Council said tankers with clean water were available, and would stay in Akaroa while the boil water notice is in place.