E coli has been detected in a private water supply on the outskirts of Masterton and about 100 residents have been warned to boil their water.
The water is only supplied to members of the Fernridge supply scheme, who have tanks on their properties in the Upper Plain area.
Grant Allan, who is on the scheme's management committee, said the bacterial level detected was low, and it was the first test, so notifying residents was not a requirement.
But after the Havelock North campylobacter outbreak, he said, the committee decided to take immediate action.
"We received advice of the test result in the afternoon and by early evening we had notified all the members of our small voluntary water scheme.
"We will be able to identify as a result of the further testing we've carried out the problem, or the cause specifically, fairly quickly.
Mr Allan said results from a second round of tests were expected tomorrow.