About 8000 residents in the Canterbury town of Rolleston are being advised to continue to boil drinking water.
The notice was put in place on Sunday after high levels on E.coli were found in a bore that feeds the town's supply.
Test on Monday were negative, but the Selwyn District Council says three days of negative test results are needed to confirm that the water is safe to drink.
Water asset manager Murray England said residents should continue to boil drinking water until the notice is lifted, as the council can't guarantee it is safe.