Cook Islanders will die if their water isn't disinfected according to government and independent experts.
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The Cook Islands News reported health experts have stepped up warnings about the need for a disinfection tool like chlorine to combat the dangers of waterborne illnesses.
A group, Te Vai Ora Maori, opposes the use of chlorine saying the water doesn't need disinfecting as it is mountain-fed and there has not been enough consultation.
However, the Secretary of Health said people can no longer ignore a decade of water test results.
Josephine Herman said results demonstrate bacteriologically-contaminated water supplies that expose people to the risk of becoming sick or dying.
Pipeline agency Te Mato Vai has confirmed it's ordered polyaluminium chloride as a coagulant, to clean water of small particles of dirt and the protozoa that cause diseases like giardia.
While the Government has not yet ordered chlorination chemicals for water disinfection, the paper reports it's expected to sign off on it this month.
Dr Herman said chlorination is an established, cost effective method for water disinfection for large water supplies including in New Zealand and Australia.
The Auckland University Medical School's Mark Thomas said there is no evidence that chlorination poses any risks to consumers.
"Sooner or later there will be significant contamination of the Rarotongan water system, that may only last a matter of a few days, or may last longer," he said while in Rarotonga.
"If a water treatment programme is in place, then the number of people who will be affected will be very much less."
Te Mato Vai project management unit said the chlorination chemical it purchased was standard practice.
"Most developed nations use coagulants as part of water treatment for water that comes from streams, rivers or lakes," said spokesperson Kate Woodruffe.
The use of chlorination for coagulation to remove dirt and protozoa was different from using it for disinfection, to kills bacteria and viruses, she said.
Polyaluminium chloride was selected for Rarotonga because it was a safer option than the alternative, alum, and because it has a longer shelf life and performs better in water quality tests. "It is essential to the water treatment process," she added.