Further testing is being carried out in the south Wairarapa town of Martinborough after E coli was detected during routine sampling of its water supply.
It is the second time in two months that such a notice has been issued.
A boil water notice was issued in February shortly before one of the district's most popular events, the Martinborough Fair, and lasted for more than two weeks.
South Wairarapa mayor Viv Napier said it was too early to say if the two incidents were linked.
She said residents should continue boiling water for food preparation and other tasks.
"Brushing your teeth, drinking, with little kids, babies, anyone who's sick, best to use hand sanitizers for washing your hands, those sort of things as well."
Ms Napier said the results from the tests would not be available until tomorrow.
A mineral in the town's water means it cannot be treated to kill germs, in the same way that is done in neighbouring towns.
Ms Napier said the water cannot be chlorinated like it is in nearby Greytown and Featherston.
"Because of the manganese in the water there is a reaction when you put chlorine with the manganese, so we are looking at what we can do into the future."
Adding chlorine would cause a chemical reaction that makes the water brown and murky, Ms Napier said.