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.
Unlike Martinborough, which suffered similar scares two years ago, Carterton's network has been chlorinated for some time.