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.