Two older people are critically ill with a gastro illness that's continuing to affect hundreds of people in Havelock North.
On Friday an elderly person died in a nursing home from gastroenteritis and authorities said the death needed to be investigated before it could be linked to the outbreak.
Schools in Havelock North reported as many as 20 percent of their pupils were absent because of sickness, and medical centres were busy.
One of the two critically ill older people was admitted to Hawke's Bay Hospital on Sunday and the other on Friday. Both are from Havelock North.
Hawke's Bay District Health Board medical officer of health Nick Jones says the type of bug they have has not yet been determined but it's likely to be related to the gastro outbreak.
Seven people had tested positive for campylobacter from stool tests by Saturday.
On Sunday, Dr Jones said the majority of the illness being reported was campylobacter.
Campylobacter causes campylobacteriosis, a gastrointestinal infection with symptoms of diarrhoea, vomiting, fever and pain. Symptoms can last up to 10 days but people usually get better without antibiotics.
On Saturday, Hastings District Council Mayor Lawrence Yule said chlorination of water "fixes" campylobacter.
"I am charged leading this organisation to deliver safe drinking water and clearly we haven't," said Mr Yule. "We have hundreds of people sick from drinking tap water and, while it's not my personal responsibility, I do lead the organisation that is charged with doing that.
"I apologise to the people of Havelock North for the distress and sickness that has been caused. It's not a pleasant bug as I've had it myself."
Chlorination of the water supply seems to have been effective and is now throughout the Havelock North water supply, Dr Jones said on Sunday.
"The boil notice will remain until we are confident there is no other bug resistant to chlorination in the water, which is expected to take several days," Dr Jones said.
Mr Yule believes the council did all it could.
"We've done all we can to communicate as quickly as we can what we're doing, what people need to do and why they need to treat their drinking water."
It is suspected that heavy rain last weekend may have caused a contamination of the Havelock North water supply.
But regular tests were clear on Tuesday. 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.
On Sunday health authorities provided advice to people to help contain the outbreak.
Hands need to be washed thoroughly by using plenty of soap, cleaning under fingernails, rinsing hands well and drying on a clean towel.
They need to be washed before and after preparing food, after going to the toilet or changing a baby's nappy, after caring for people with campylobacter and after playing or working with animals.
Children and older people are most at risk of dehydration, and fluids - while the diarrhoea lasts - are important.
Mr Yule said the outbreak 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."
NZN / Newshub.