The cause of an E. coli outbreak in Lake Hawea remains unknown.
The Queenstown Lakes District Council was informed of E. coli readings in the Hawea water supply network after it was detected during routine testing on January 18.
A boil water notice for Hawea residents was lifted on January 22.
In a letter to the Guardians of Lake Hawea, QLDC chief engineer Ulrich Glasner said attempts had been made to find the cause of the contamination.
The reservoir had been inspected to see if an animal could have entered, and construction work on the water network had been ruled out as having caused the contamination, he said.
