As many as 100 children were off school in Havelock North today due to a stomach bug which may have originated in the Hawke's Bay town's water supply.
Te Mata School reported 120 kids off sick today, about 100 due to illness.
"Of the 100 approximately 94 are due to some kind of stomach upset," a post on the school's Facebook page read.
And Hereworth School reported several students unwell over the past few days - with symptoms including vomiting and diarrhoea.
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"To avoid further spread of this illness within the school please follow the Ministry of Health guidelines and keep your boys at home for 48 hours after the last episode of vomiting or diarrhoea," the school wrote on Facebook.
Tonight, the Hawke's Bay District Health Board urged residents to boil water.
Medical officer of health Nick Jones said while the source of the outbreak hadn't been confirmed there was a possibility that the cause of the illness was from the water.
"We want to prevent any further illness, which is why we are taking such significant precautionary measures and encouraging everyone in the Havelock North area to boil water over the weekend or until the cause of the outbreak was determined.
"Boiling the water was the only certain way to kill all the bugs that might be causing the illness," Dr Jones said.
Hastings District Council had also chlorinated the water this afternoon as another precautionary measure. However until the type of infection had been determined it was recommended to take the extra step and boil water, for a minute, before drinking it.
The institute of Environmental Science and Research (ESR) would be working over the weekend to identify where the contamination had come from and what the illness was caused by. Potentially the significant rain event from last weekend may have contaminated the supply and further testing was underway.
Dr Jones said as the illness was widespread in the Havelock North area it was unlikely to have come from a food source and water borne illness was more likely, however testing would determine the exact cause.
Symptoms of the illness being reported were diarrhoea and influenza like symptoms; headaches, muscle pain, fever and feeling generally unwell. Symptoms could last up to 10 days but would usually get better without antibiotics.
Children and older people were most at risk of dehydration and fluids while the diarrhoea lasted was very important.
In September last year, low levels of e-coli were found in the town's drinking water supply.
The bacteria was combatted with chlorine treatment, with the Hastings District Council reporting it was possible that heavy rain of the previous few days had led to a bore being contaminated by storm water.