Hastings District Mayor Lawrence Yule has publicly apologised over what has been described as a "horrific" outbreak of waterborne illness in Havelock North.
Up to 2000 people have fallen sick after the town's water supply was infected with campylobacter bacteria.
All of the town's schools will stay closed for the next two days, and many residents are angry they were not warned earlier their water was contaminated.
Age Concern board member Wayne Bradshaw said the Hastings District Council should have been more prepared.
"They knew about it midday on Friday. It didn't get it into that [public] arena until seven o'clock and that was only through press release. Not everyone has computers, not everyone looks at computers at seven o'clock on a Friday night," Mr Bradshaw, who also sits on the council, said.
"They didn't ring up the rest homes as they should have. If they'd responded sooner, at say one or two in the afternoon, they could have rung up the rest homes, rung up the schools, rung up the childcare centres - they could have said 'look, we've got a problem and you need to know about it'."
The council said it would hold an independent inquiry into how the contamination happened.
Mr Yule told Checkpoint with John Campbell it was the largest outbreak of its kind in New Zealand history, with at least 1000 to 2000 people affected.
"First of all, as the mayor, I apologise to the people of Havelock North for what has happened," he said.
He said he was told about the contamination about 3.30pm on Friday, and a decision was made then to chlorinate the water.
About five hours earlier, there had been a suspicious test, he said - but the council had to be sure something was wrong.
"On Tuesday, we conducted a test; it was confirmed that the water was clear in Havelock North.
"On Thursday, we conducted a test and as part of these 48-hour tests - they give you an indication at 24 hours - on Friday morning, from the Thursday test, there was an indication there was something abnormal in the water.
"We notified the DHB, who then rang around all the schools to see if there were any schools in the area with sickness. It was then clear that there was a number of children unwell in Havelock North. So the decision was made at three o'clock 'let's chlorinate the water, not wait until the following day', which was normal practice, on the basis that something was going on here.
"They then decided at half past five as a further precaution - because we don't know what's going on here and we don't know what the cause was - that we issue a 'boil water' notice.
"The decision was sort of publicised from about six o'clock."
He said the standard drinking water test was every two days, and that process continued after Tuesday because there was no sign of a problem at that point.
He said he thought the municipal water supply was the cause of the illness.
"We will get the final result of what's in the water [tomorrow morning] - an indicative result I should say because nothing's ever final. What people don't understand is that most of these bugs have to be grown - so if you find something of indication you have to actually grow these bugs to see if there's enough to be a problem."
He said the next concern was finding out whether the aquifer itself, with water about 50 years old, was contaminated.
"We're going to do a major investigation into what caused this because it can never happen again. And we're going to put processes and systems into place to ensure that."
Prime Minister John Key said what had happened was unacceptable, and the Ministry of Health might hold its own inquiry.
He would be briefed by officials tomorrow, he said.
"Fundamentally we need to understand how it took place, because for the number of people that are now sick it's very clear, given the dilution that you would have expected to take place and the size of the water source, that quite a lot of material has got into the water source."
He would not comment on the efficiency or quality of communication from the council with the public until an investigation was complete.
Associate Health Minister Peter Dunne told Checkpoint the problem had started to become apparent on Thursday or Friday.
"It's pretty horrific at the moment. I think it's unfortunate, but we're working as closely as we can with the DHB and the council to resolve it.
"On the face of it, when you look at the numbers coming through, it's got huge risk potential, particularly since we may not know for up to 10 days what the level of infection is because of the particular bacteria and the way it incubates."
School closed as outbreak worsens
The Hawke's Bay Regional Council said presentations of seriously ill people to accident and medical centres had continued to increase.
Yesterday, 183 people presented to a GP - and Hawke's Bay Hospital said it had 11 people present to it overnight, with two of those admitted.
The hospital had 19 patients admitted into it for further care, and two people remained in a critical condition in intensive care.
The council said all the results back from those with the bug were positive for campylobacter.
It said the "boil water" notice would remain in place until it was confident there was no other bug resistant to chlorination in the water, which was expected to take several days.
All of the schools in the town have been shut down for the next two days.
Havelock North High School principal Greg Fenton said the decision was made at a crisis meeting with primary, intermediate and high school principals and the Ministry of Education early this afternoon.
Mr Fenton said the closure came under Section 65 of the Education Act, which provided for schools to close in emergency situations.
On Friday, 25 percent of the high school's students called in sick - and, today, that had increased to about half, he said.
Students who had made it to school were starting to get sick, and were being sent home, he said.
The schools closed were Havelock North High School, Havelock North Intermediate, Te Mata School, Havelock North Primary School and Lucknow Primary School.
Two other private schools - Iona College and Woodford House - closed earlier today, and were also not expected to reopen until Wednesday.
Residents angry over communication
Denton Peak Pharmacy manager Robyn Wood said the outbreak was the worst of its kind she had experienced.
Doctors rang her on Sunday to open up to provide fluid replacement products, she said.
"Yes, this was a very bad outbreak. Nothing that I've seen before in my career, so we were very very busy."
Some residents told RNZ they were still nervous about drinking the water, despite the Hawke's Bay DHB saying chlorination seemed to have been effective.
Colleen Pascoe said her daughter was very unwell, and local authorities should have spoken up sooner.
"To leave it so long and have everyone in Havelock be sick is just disgusting," she said.
"My daughter is so sick... She can't walk, her hips are so sore."
Havelock North mother Kayla Vivian said her seven-year-old son started getting sick last Monday or Tuesday.
"We started with a headache, vomiting, fever. The headache has got worse and worse and worse. We finally stopped vomiting, stopped headaches, stopped the fever and now we've had non-stop diarrhoea that has got to the point where I've had to throw household stuff out, the washing machine has been non-stop."
She said she was out of pocket from not being paid because she had only recently started working and had now to stay home to look after her son.
One resident said the communications from council had been the most worrying.
"We had no communications until later on Friday night, when apparently they knew about it earlier on Friday morning, so we could have stopped our children from drinking the water, stopped elderly from drinking the water.
"My little baby she was quite sick over the weekend and it's a real concern as a parent as to why we weren't communicated [with] earlier."
Another woman said her elderly neighbours had fallen ill and nobody had told them about the contamination issue by mid-morning on Saturday.
It is the third case in three years of bacterial contamination in Havelock North's water supply coming from bores, after E coli was discovered in the water in 2013 and 2015.