Queenstown's mayor says putting filters on regional water sources is expensive.

The area is in the grips of an outbreak of infections from a waterborne parasite.

Known cases of cryptosporidium illness have reached 21 - and the boil water notice could last for months.

The national water regulator has slapped a compliance notice on the region.

Mayor Glyn Lewers says there are a lot of water intakes, and six don't have filters.

"There is a cost element to it but there is a progressive program at work, and we try to manage those costs because of the rate burden on our ratepayers."

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