A possum in the Lake Hawea reservoir has led to "emergency chlorination'' being put in place for a small part of the township's water supply.
Queenstown Lakes District Council chief engineer Ulrich Glasner said that routine testing showed elevated E. coli counts yesterday morning.
An inspection found a possum floating in the reservoir.
"How it entered the closed system is unclear,'' Mr Glasner said.
A "small number'' of Hawea residents would notice their water being chlorinated for the next few days, while the council and the council's water maintenance contractor, Veolia, investigated how the possum got into the reservoir.
Because most Lake Hawea properties could be supplied directly from the town's new borefield, bypassing the reservoir, the chlorination would only affect the Lake Hawea Holiday Park and properties on the west side of the dam.
The town's water system is only chlorinated when E. coli has been detected.
The last time that occurred was in January.
Mr Glasner said the community had been very firm about not wanting routine chlorination, and this was reiterated at a public meeting last month.
Veolia was flushing the reservoir yesterday and would then empty it for a detailed inspection next week.
Mr Glasner said chlorination would continue until the council was confident all extraneous material had been removed from the reservoir and the possum's entry route had been identified and blocked off.
