An e-coli indicator has been found in one of the nine water tankers used to supply drinking water to Havelock North residents.
The tanker had been filled up in Hastings. Hastings mayor Lawrence Yule said a decision had been made to chlorinate the Hastings water supply immediately.
The suspect tanker was parked in the Havelock North High School car park. Residents who took water from this site and still have it in containers are asked to dump it.
Residents were directed to the tankers by the Hawke's Bay District Health Board after thousands became ill from campylobacter in the water last week.
The water from the other tankers has been tested and is clear, however the water in all the tankers is now being chlorinated.