Outbreaks of waterborne illness will become more common if no effort is made to protect the quality and sustainability of New Zealand's environment, a public health professor warns.
More than 5000 people fell sick with gastrointestinal illness when Havelock North's drinking water became contaminated with campylobacter bacteria in August - over a third of the town's population.
Last month two Havelock North residents were diagnosed with Guillain-Barré syndrome, a rare neurological disorder linked to the illness.
An independent inquiry is now investigating the cause of the outbreak, which led to the biggest mass poisoning in New Zealand's history.
But University of Auckland professor of public health Alistair Woodward told Sunday Morning the inquiry needs to be broadened to consider not just the immediate causes, but other factors putting New Zealand's water supply system under stress.
"The independent inquiry is an opportunity to take stock. It would be a pity if the inquiry focused only on the immediate factors - although it's important that we know whether there were mistakes made or simple things that could have been done to avoid this particular espisode," he said.
"But the bigger question, and the one we really need to grapple with, is what is it in the New Zealand environment that makes these things happen."
Dr Woodward, who is head of epidemiology and biostatistics at the university, said although there have been several recent outbreaks of water-borne illnesses throughout New Zealand, they had not been on the same scale as the Havelock North situation.
He said a growing population, which also contributes to the increasing drawdown of water supplies; increasing industrial and agricultural activity; and climate change were all having an impact on the water system, and could be contributing to the outbreaks of illness.
"The expansion of agriculture has been a particular issue."
In order to prevent or reduce similar outbreaks in the future, those factors needed to be examined and addressed, he said.
"If we don't, the danger is with Havelock North, then the chances of something like this happening again are high."
Climate also had a major effect, he said.
"The outbreak in Havelock North was preceded by the heaviest daily rainfall in 10 years, which came after a period of relative dry, so there was very heavy run-off... We know from overseas, that is a real danger signal for water supplies."
New Zealand was facing a four-fold increase in heavy rainfall with climate change, and defences needed to be ready to deal with the risk of another large outbreak, Dr Woodward said.