A death in a Havelock North rest home may be linked to a waterborne disease that has caused hundreds of children to miss school.

Hawke's Bay District Health Board chief executive Dr Kevin Snee said it became aware of a number of cases of gastroenteritis in the Hastings suburb, particularly amongst school children, on Friday.

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People were also visiting GPs and Hawke's Bay Hospital's Emergency Department.

An increase of cases was seen overnight and into Saturday, some through St John ambulance and there were eight "quite unwell patients" in one residential-care home.

"We have also had a death in a nursing home, which may or may not be related to this problem, but it is a consequence of a gastric-like illness," he said.

"We have also seen in the hospital 18 patients today and admitted eight."

Extra help was being provided to rest homes "and the hospital itself is well prepared and coping well."

Because campylobacter was detected in patients and the nature of distribution the Hastings suburb's water supply was believed to be the source.

After meeting with Hastings District Council, responsible for the water supply, it was decided to chlorinate the Havelock North water supply from two bores.

One bore tested positive for E-coli.

He said chlorination should kill most water-borne diseases and boiling would make it safe. Chlorination would continue and water boiling recommended until the cause of the incursion identified.

Hastings District Health Board Medical Officer of Health Dr Nicholas Jones said patients tested positive for campylobacter, which was usually spread through water or food.

Symptoms ranged from flu-like symptoms to headaches, muscle pain, fever and feeling generally unwell. Symptoms could last up to 10 days but would usually get better without antibiotics.

Hastings mayor Lawrence Yule said the cause of the incursion was a mystery, happening after testing on Tuesday which found the water bacteria free.