More than half of Havelock North households have been affected by gastro illness outbreak , says Hawke's Bay District Health Board (DHB) chief executive officer Kevin Snee.

Based on a telephone survey he said 3200 people were affected.

Speaking at a press conference today Hawke's Bay District Health Board chief executive Dr Kevin Snee said the number of cases appeared to have plateaued, with St John ambulance reporting fewer calls related to gastro illness.

The Hastings suburb has about 13,000 residents.

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Medical Officer of Health Nick Jones said the DHB's reaction to possible waterborne illness exceeded New Zealand Water Standard requirements.

"It is a major decision to put the reputation of a water supply at risk when you don't have absolutely convincing evidence that there is a problem," he said.

"We understand from [Hastings] Council that 80 per cent of the time when those initial samples come through showing presence they turn out to be not significant.

"Nevertheless we felt sufficiently concerned it was important to go ahead and actually get that water chlorinated.

"I am absolutely convinced that has saved a lot of people's health."