The Queenstown Lakes District Council has apologised to Lake Hawea residents for not giving notice that chlorine was to be added to their water supply on July 18 to address a bacterial contamination issue and biofilm build-up in some pipes.
Residents have complained about the lack of notice and the strong taste and odour of the water.
The Lake Hawea Residents Association has sought an explanation.
Acting water services manager John Porter said in a statement yesterday the contractor, United Water, considered it was acting in the best interests of the community in chlorinating the supply after discovering e-coli in a sample taken from a tap outside the toilet block near the Hawea Motor Camp on July 18.
Mr Porter said the contractor did not know about a protocol to warn the community of chlorination.
United Water has agreed to inform the community before chlorinating the supply in future.
After complaints were received, it flushed the system and will continue to do so, as required.
The company's report released by the council on Thursday reveals the emergency chlorine dosing system at the Hawea intake had not been operating for some time.
"UW was not aware of the deep community concerns regarding chlorine and its preference for no remedial chlorination in the case of microbiological failure.
However, it should be noted that United Water would always strongly recommend remedial disinfection of any distribution in the case of an e-coli isolation, and the use of chlorine is the most efficient process," the report states.
The United Water report confirmed tests throughout the chlorination period indicated the level of chlorine remained within acceptable drinking water standards.
The highest reading indicated 1.69mg per litre.
The Ministry of Health maximum allowable value for drinking water is 5mg per litre.
The report also showed the effect of chlorination was significantly worse for residents who had a build-up of biofilm [a coating of microbes] in pipes.
"We certainly apologise to the community for the failure to issue a chlorination warning and it is unfortunate that the biofilm has had an added effect on the supply and the community," Mr Porter said.
The contractor had recommended a six month air scouring regime to address biofilm build-up, which the council would discuss with the community, Mr Porter said.
The council would also investigate the cause of the original e-coli contamination.
