The gastro bug that's made hundreds of people sick in Havelock North, and potentially killed an elderly person, is thought to be from the campylobacter bacterium.
Hastings District Council Mayor Lawrence Yule told NZ Newswire it won't be confirmed until next week so the boil-water notice will remain in place until Wednesday.
Most ill people are visiting GPs but 18 people have presented at hospital and eight have been admitted.
Seven people have tested positive for campylobacter from stool tests, he said.
Chlorination of water "fixes" campylobacter, and "boiling fixes anything else", he said.
The council needs to confirm "it isn't anything else" before it lifts the boil-water notice.
A person has died at a nursing home from gastroenteritis but the source isn't clear because there have been outbreaks of airborne disease that could have caused it, Mr Yule said.
Tests of the Havelock North water supply were clear on Tuesday, but on Friday one result of tests from the day before indicated the presence of E.coli, which is not itself a harmful bacterium but is a marker for others.
Campylobacter causes campylobacteriosis, a gastrointestinal infection with symptoms of diarrhoea, vomiting, fever and pain.
Mr Yule said Hawke's Bay District Health Board picked up that there were large numbers of people falling sick late last week and worked with the council.
Te Mata School reported 94 pupils absent on Friday due to a stomach upset and other schools said as many as a fifth of pupils were sick.
"If we can prove that it is only campylobacter then we may be able to withdraw the boil-water notice early because the chlorine will deal with it," Mr Yule said.
Residents took to social media to vent frustration.
"Boil your water Havelockians. I went down on Wednesday and I wish the symptoms on no-one," Caleb Milne wrote on Facebook.
Bree Jardine added "Not a nice one. Trust me".
Local Bridget Jarvis says the Hastings District Council and the Hawke's Bay DHB should have acted sooner.
"It took them so long for any communication to come out about what had happened, that the water might be affected" she says. "It's Saturday now. You know, the storm started a week ago."
Ms Jarvis says it's not the first time the water has been infected.
"It happened in September last year. I don't think it was as bad, but you'd think that they would be so onto it with the testing knowing that water system isn't up to par.
"I sent out a mass text yesterday when I heard via this doctor that the water might be contaminated, and I got feedback from so many people. Probably 90 percent of people had someone affected in their family."
There was a run on bottled water at the New World supermarket in Havelock North and it got in extra supplies that would usually meet demand for six months.
There is speculation rain last weekend may have tainted the water supply.
Mr Yule said it was a mystery because the water test on Tuesday was clear.
"We have three bores. One has had E.coli in it before in the last year and we found that and no one got sick and we treated it. We shut the bore down.
"The bores that have been running this week have never had E.coli in them ever and that's part of the mystery."
The Ministry of Health is aware of the situation and has been involved in dealing with it, he said.
NZN / Newshub.