Carterton's E coli readings are proving a mystery to water boffins.
An on-off boil water notice has been in place since March 12, after low-level readings of the deadly bacteria.
Carterton District Council (CDC) received two other positive readings over the summer.
The most recent results have left experts baffled.
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Advertise with NZME.Unlike Martinborough, which suffered similar scares two years ago, Carterton's network has been chlorinated for some time.
CDC infrastructure services and regulatory manager Dave Gittings said E coli contamination in a reticulated, chlorinated water system was extremely rare.
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"There are many excited scientists around New Zealand who are looking into writing a paper on this," Gittings told the council's infrastructure committee on Wednesday.
"This is one of the very few instances around New Zealand that you will find a reticulated system, with highly residual chlorine, that has an E coli count in it.
"That is extremely unusual. They're excited. I'm not so excited."
Committee chair Russell Keys acknowledged the team's work to try to get to the bottom of a situation that was "extremely frustrating for yourselves and for the residents".
"Especially having been picked up for the third time.
"We've taken a responsible attitude. We're going to have to do these notices for the safety of our community.
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Advertise with NZME."And that's going to be uppermost in our mind. We've got to make sure that no one in the community gets ill over this.
"If it was a simple thing that we could just track down, I'm sure we will have it done."
Mayor Greg Lang said the council and experts were "doing the best that we can".
"There's very low level, and it's a responsible approach that we've taken to deploy that.
"We understand it's inconvenient for many members of the community, but the community's health is number one."
CDC expects to issue a further situation report on Friday.
