The national water regulator estimates hundreds of water treatment plants in New Zealand serving about 10 percent of the population do not have filters to keep protozoa out.
Several councils and government ministries could be facing bills in the hundreds of thousands of dollars to fit essential filters to their drinking water supplies in the wake of Queenstown's cryptosporidium gastro outbreak.
There are 60 confirmed cases of the parasite-induced gastro, that can cause severe diarrhea, stomach cramps and vomiting.
National water regulator Taumata Arowai has issued notices for 150 water treatment facilities, run by councils and government departments.
Taumata Arowai head of regulatory Steve Taylor told Checkpoint barriers were necessary to stop protozoa - organisms that can cause a raft of health issues - from entering drinking water.
"That's something that's quite clearly set out under the drinking water rules and standards."
He could not say how many of those suppliers were not compliant but said it would be in the hundreds.
It was a "significant proportion", and about 10 percent of the population "don't have filters", he said.
"To say that [the] water is unsafe is probably not necessarily the case. The purpose of those filters is ... to avoid these sorts of contamination.
"So without those filters, there's certainly a really high risk that that water will become contaminated. It's not a situation we want."
He was reluctant to name the non-compliant water treatment plants.
"We're going through ... verifying with each supplier now. We've sent a letter to them last week setting out really clearly the expectation they must comply with these rules.
"We've also asked them to clarify for us the steps that they're undertaking and the circumstances around how they're treating water, how are they managing risks in their network, and how are they monitoring and ensuring that those risks are being effectively managed."
Taylor said suppliers had until Wednesday 4 October to provide an update to Taumata Arowai on the filters.
"Once they've confirmed that tomorrow we will release that information publicly. Not only that, we'll be sending a letter to each of those suppliers setting our expectations for compliance."
He said the information would be made public by late tomorrow evening.
The cause of Queenstown's cryptosporidium gastro outbreak has yet to be identified but it has been revealed some of the networks that supply the town with water are not fitted with the compulsory filter.
A boil water notice has been in place since last month.
But residents and tourists may be able to drink from taps as soon as Thursday.
