Hawke's Bay District Health Board and Hastings District Council are strongly refuting reports their reaction to the gastro outbreak were too slow.

"If people think that we were sitting around watching numbers of people getting ill and not doing anything, they are seriously incorrect," Medical Officer of Health Nick Jones said.

More than half of Havelock North households have been affected by gastro illness.

Based on a telephone survey of 254 households the DHB said it estimated 3348 people were affected by the outbreak, of the suburb's 13,000 residents.

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DHB chief executive Kevin Snee said the number of patients appeared to have plateaued, with St John Ambulance reporting fewer calls. One of two people still remained in intensive care. Thirteen patients were still in hospital and the emergency department had 10 gastro-related presentations yesterday.

Test results from a person with a gastro illness who died were not yet available.

The public notice to boil water remained in place in case cryptosporidium was present but it was so far undetected in water or patients. The bores which service Havelock North, and where the contamination started, were being tested twice a week.

Although an indication of any harmful presence is provided at 24 hours, 48 hours are needed before a confirmed result. Last Tuesday's test returned both a negative indicator, and negative result.

Despite rumours, it was not until 9.53am on Friday that council became aware of any problems with the bore, when a positive indicator was returned.

The first to know were the council's water treatment operators, water services manager Brett Chapman said. The DHB also only became aware of a possible waterborne outbreak on Friday.

Normally, the council would wait the full 48 hours before taking any further action as about 70 per cent of the tests end up as false positives.

However, just before 1pm on Friday, the council were contacted by the DHB, and told there were a number of people presenting with gastro illness. There were also hundreds of children off school ill.

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An hour later, teams from council and DHB met, and by 3pm the decision had been made to chlorinate the water.

Mr Chapman said they had been informed of the DHB's concerns around the cases of illness they were starting to see.

Based on the positive indicator, a precautionary measure was taken immediately to either eliminate the bug, or ensure any further contamination could be suppressed.

Mayor told at 2.30pm

Hastings mayor Lawrence Yule was not part of these discussions - he was informed of the situation at 2.30pm by council chief executive Ross McLeod, who had been informed just after midday.

He had been advised of the indicator test, which did not confirm anything, but suggested there was something abnormal.

"I was also told that there was a number of people being reported ill ... I had no idea the scale of the number of people being ill until much later in the afternoon, and into the evening," he said.

He was informed about the meetings going on, and at 4.30pm told the decision had been made earlier to chlorinate the water, and that there were discussions about whether a boil water notice should be issued.

"[The chlorination] effectively started on the ground at 5 [pm] once they got the chemicals and by 8pm the whole system had chlorine through it, and our experts view is that at that stage it had killed all the bugs which were now present," he said.

The decision to issue boil water notices happened around 6pm, and media were notified around 6.30pm.

Mr Jones said the public notice to boil water was an added measure in case cryptosporidium was present but it was, so far, undetected in water or patients.

The boil water notice would remain in place until the bacteria's presence could be "completely ruled out". Mr Yule said, "what we're dealing with here is a contamination that has been going on for a number of days before its been picked up," he said.

"It's a bit frustrating for us, I want to absolutely, categorically, say we did not know about this until Friday morning.

"I get the fact that actually it's a very emotive issue, people have been extremely unwell, they're looking for somebody to blame, they're thinking 'actually if we'd known earlier we might not have drunk the water and we might not have gotten sick'."

However, Mr Yule said the council had acted faster than they normally would, as they thought something was wrong and wanted to act quickly, rather than wait for a final result on Saturday.

DHB insists actions exceeded requirements

Mr Jones said the DHB's response exceeded Drinking-Water Standards for New Zealand requirements.

"It is a major decision to put the reputation of a water supply at risk when you don't have absolutely convincing evidence that there is a problem."

Despite the indicators often returning false positives, Mr Jones said, "nevertheless we felt sufficiently concerned it was important to go ahead and actually get that water chlorinated.

"I am absolutely convinced that has saved a lot of people's health."

On Thursday, the DHB had been provided with a positive indicator test from a private water tanker driver operator from water taken from a Havelock North bore, which Mr Jones said was a "red herring".

The assumption was the water was always pure before entering the tanker and the matter was "followed through as normal procedure".

The council were not informed of this test. Yesterday Associate Health Minister Peseta Sam Lotu-Iiga said he believed an inquiry into the outbreak would likely be headed by a retired High Court judge and findings would have implications for all supplies.

He was in Hastings to see where support could be provided and said he was reassured local agencies had the necessary resources on hand.

Tukituki MP Craig Foss said the Havelock North community was "smashed around" by the outbreak. "I don't think there is a family that hasn't been affected - frazzled parents with very sick children or elderly folk," he said.

"It is not acceptable in 2016 for this to happen."

As an extra precaution, the supplies of Hastings, and Flaxmere were also being monitored.

What you need to know:

• The boil notice is still in place.
• Water needs to be brought to a rolling boil for at least one minute.
• Electric jugs with a cut-off switch can be used as long as they are full - allow the water to come to the boil and switch off.
• Boiled water should be covered and allowed to cool in the same container. The taste will improve if allowed to stand for a few hours before use.
• Hands need to be washed thoroughly by using plenty of soap and warm water, cleaning under fingernails, rinsing hands well and drying on a clean towel.