E-coli has potentially affected all of Havelock North's water supply and the Hastings District Council (HDC) is combating the outbreak with chlorine.
HDC Water Services manager Brett Chapman said low-level microbial contamination was detected from routine monitoring.
"Most supplies in New Zealand dose chlorine on a continuous basis which acts as the first barrier against this type of contamination," he said.
"Nonchlorinated supplies like Hastings do not have this barrier in place, hence the need to act immediately with chlorine as a precautionary measure."
The contamination was detected in the supply last Wednesday, the services manager saying the results were confirmed with a second test the next day.
"This is the standard Ministry of Health (MoH) practice," he said.
"E-coli is the indicator organism that the MoH requires to be monitored for in drinking water supplies.
"The whole of the Havelock North supply is potentially affected."
People might notice the taste of their water was slightly different for a few days due to the introduction of chlorine. Those opposed to the flavour could get free nonfluoridated water from a site near Hastings Library in Eastbourne St, Hastings, which is clearly signposted.
While the probable contamination was widespread, Mr Chapman said the levels detected were very low, at 3.6 colony-forming units per 100mls (3.6cfu/100mL), but still required a response.
Levels of between 1 and 10cfu/100mls present a very low risk to human health.
While the council has not confirmed what was causing the contamination, staff were investigating.
"The circumstances this time confirm that it likely originated from one of the Havelock North bores and this has been shut down," Mr Chapman said.
While this happens, the Havelock North water supply will be treated with chlorine at a dosage of one part per million (ppm) at the bore which achieves around 0.3 ppm in the reticulation network. "The maximum treatment permitted under drinking water rules is 5ppm within the reticulation," he said.
This is not the first time HDC has had to take such action, Mr Chapman reporting a similar occurrence in July 2013.
"We were unable to identify the source of contamination on that occasion," he said.
Council tests the water at a six-day frequency across a range of sites. More results should be available today.
A post on the Hastings District Council Facebook page announcing the chlorine introduction got people talking - attracting about 30 comments, 40 "likes" and 59 shares.
Most people were simply happy to be informed of the situation, some simply wanted to ask questions others had already noticed the change.
"I could smell it [chlorine] when I turned the shower on this morning. Its potent," one commenter wrote.