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Chlorination of the Lake Hawea water supply is set to continue as the contamination source of the town's water system remains a mystery.

The town's water supply continues to be treated with chlorine more than eight months after it was put on a three-month temporary treatment at the end of last year.

The Queenstown Lakes District Council's chief engineer Ulrich Glasner said the water supply was being monitored and treated as a precaution, to ensure safe drinking water.

That approach was supported by the Southern District Health Board which was responsible for regulating the council's water management, he said.

As well as Lake Hawea, the water supplies in Glendhu Bay and Arrowtown were also being temporarily chlorinated, at a cost of about $4.10 per household, he said.

Earlier this month, a letter was sent to some Lake Hawea residents outlining why the treatment was ongoing.

In that letter, the council's acting general manager for property and infrastructure, Myles Lind, said monitoring had picked up issues at multiple sites around the town and risks were not isolated to one or two pipelines.

The council would continue to investigate the water network to find the sources of the contamination and was regularly flushing the town's storage reservoir, Mr Lind said.

Typically a reservoir would only be cleaned once every 10 to 15 years but the Lake Hawea reservoir had been completely cleaned three times since 2011, to remove sediment.

A physical cut into the network was also being planned, to gain more insight into the organic matter attached to the surface inside the pipelines.

Better control of the number of connections being made to the water network, both illegal and legal, was also being worked on.

The council did not know how many illegal connections there were.

Despite the problems, Mr Glasner said the town's water infrastructure was up to standard.

The Hawea Water Safety plan was being worked on and was required to be reviewed, updated and submitted to the district health board for review and approval before the end of the year, Mr Glasner said.

Last month the council put on hold plans to chlorinate all untreated public water supplies in the district after it attracted strong opposition from some residents.

Because of the response from the affected communities, the council deferred the plan to allow time to seek additional information before a decision was made.

Hawea Community Association chairman Paul Cunningham said the association's opposition to the permanent chlorination of the town's water supply had not changed.

Mr Cunningham said there was obviously an issue with something in the water network and the council was still trying to find the cause. All residents could do in the meantime was hope for it to be sorted as quickly as possible.

''We've just got to run with it and hope it comes right.

''But the community association's position remains the same, we're against permanent chlorination and the sooner they can get the water clear again, the better.''

tim.miller@odt.co.nz