Relief water for the campylobacter outbreak in Havelock North was possibly contaminated and all Hastings water chlorinated yesterday.

Water from the tanker stationed at Havelock North High School since Monday, failed an indicator test for bacteria and was removed.

It was taken from a bore tested for clean water but Hastings District Council chlorinated all Hastings water in case the tanker water was found to be infected.

Drinking water assessor Peter Wood says the indicator test could be a false positive due to the difficulty of getting a clean in-the-field sample.

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The tanker was one of nine supplying drinking water to residents after confirmation of bacteria in the Havelock North water supply on Saturday morning.

Yesterday the Hawke's Bay District Health Board had 87 confirmed notifications, and 250 probables for a total of 337.

Hospitalisations are sitting at 16 in general wards, and one patient remaining in ICU. There have only been three recent presentations at ED, with 138 to GPs. These numbers continue to move downwards, with five calls to St John and five reports in aged residential care facilities. Hawke's Bay District Health Board chief executive Kevin Snee said there was a reduction in new cases of campylobacter. Cases of Hastings residents were due to secondary infection or visits to Havelock North. Results from the deceased's campylobacter test are expected today. Hastings District Council water services manager Brett Chapman reassured residents there was no physical connection between Havelock North and Hastings water. The Hastings aquifer was more likely to be safe than Havelock North's, because it was sourced from more than 60m through five confining barriers, he said.

After the failed tanker test the council "immediately" made the decision to chlorinate Hastings water and informed the health board, which replied it was an appropriate response. Those who took water from the school were asked to dump it.

The water from the other tankers was tested clear but replaced with chlorinated water.

Hastings Mayor Lawrence Yule said it was believed the cause of the failed test was likely to be the tanker, however as a "super precautionary approach" the water supplies for Hastings and Flaxmere were chlorinated.

The daily tests for those supplies have been clear.

He said, as a precautionary approach, people could boil drinking water or use bottled water while the chlorine worked its way through the system.

"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," he said yesterday.

A positive indicator for E.coli was "a blow".

"It's an unexpected event. We're still working out how this happened. I thought from [yesterday] we would be in effectively the time when we're putting a whole lot of things right, and sorting things out, but then bang out of the blue this happened."

There was a 70 per cent chance yesterday's indicator was a false positive.

"The Hastings network has tested clear consistently. There has been nothing in the Hastings water for a long time, and all parts of the network have been safe and free of any tests positive tests since Saturday so we can't explain it."

However, as people were concerned the call was made at 10am yesterday to chlorinate the supplies of Hastings, Flaxmere, and Bridge Pa - a process expected to have been completed by 6pm last night.

"In my view and in the experts' view the risk of the Hastings supply being contaminated is very, very low but we have been super cautious and we've decided to take no risk and bring these measures in."

He said if the full results today showed an E.coli presence, then the council would work to find out how it got into the tanker.