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.
It is believed that the cause is likely to be the tanker, however as a "super precautionary approach" the water supplies for Hastings and Flaxmere are now being chlorinated. The daily tests for those supplies have been clear.
While no boil water notice has been issued, as a precautionary approach, Mr Yule said people may want to boil any drinking water or use bottled water while the chlorine works its way through the system.
He said the test was an indicator, but the council had decided to make the information public.
"We will know the full results tomorrow. Seventy per cent of these test come back negative, but we have decided as a precaution to chlorinate the Hastings water supply."