Dead animals found in a Banks Peninsula reservoir may mean residents in Akaroa and Takamatua may have to boil their water for weeks.
Christchurch City Council said several small dead animals were found in the reservoir on Tuesday during a routine check.
Testing confirmed there was enough chlorine in the water to kill bacteria, but there was still a possible protozoa risk which chlorine will not kill.
The medical officer of health said water used for drinking, food preparation, utensil washing, brushing teeth or ice should be boiled until further notice.
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The council said staff were working on a way to bypass the reservoir so it could be emptied and cleaned.
"Until that happens, the boil water notice will remain in place," said acting head of Three Waters Adam Twose.
"Bringing water to the boil is sufficient to kill bugs. If you can't boil your water then bottled water is the only other option."
Routine testing is carried out three times per week in the area.