Hawke's Bay District Health Board medical officer of health Dr Nick Jones yesterday said the majority of people presenting with the gastro illness were affected by water-borne campylobacter bacteria.
He said animal faeces may have contaminated the water supply, possibly into the groundwater itself.
Professor Nigel French, the director of the Infectious Diseases Research Centre at Massey University, said groundwater was much less likely to be contaminated than surface water.
"But if it is campylobacter, based on previous experiences, it is most likely to have come from cattle and sheep and run-off of effluent/faeces," French said.
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"However, campylobacter can also be carried by poultry and wild birds.
There was a similar outbreak in Darfield in 2012.
"I think this outbreak demonstrates that even secure groundwater can become contaminated and therefore testing and treatment is advised to ensure the best public health outcomes, particularly if there has been a high-risk event, such as heavy rainfall."
The bore that tested positive for E. coli - campylobacter tests are pending - is within sight of Te Mata Mushrooms, which uses chicken manure in its compost.
Owner Michael Whittaker said the property used shallow bores for drinking water and from its 120 staff only one was sick with gastro-illness.