An ecology professor says council measures of water quality around Hawke's Bay are lower than any he has seen before in New Zealand.

A government-led inquiry launched today will look at how Havelock North's water supply became contaminated and how the response was handled by local authorities.

An estimated 4100 people have suffered from gastric illness following the contamination of the water supply, and more than 500 have been confirmed as due to campylobacter.

Several people were hospitalised, and a coroner is looking at whether the death of an 89-year-old woman, who had contracted campylobacter, was from other underlying causes or was connected to the infection.

In the latest round of tests, Hastings, Flaxmere and Bridge Pa returned clear results meaning the water there continued to be safe to drink.

However, while the chlorine-treated water supply in Havelock North had also been cleared, a boil water notice remained there.

Massey University professor Russell Death has studied freshwater in the broader Tukituki-Papanui-Karamu area, which includes Havelock North.

He told Checkpoint with John Campbell macroinvertebrate community composition (MCI) values, which measured the general health of the water, were very low in the broader area around Havelock North.

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"A town water supply in New Zealand is infected by many of the pathogenic organisms that live in our water supplies, it's not surprising at all - in fact, it's inevitable," he said.

He said, normally, a very unhealthy river could present MCI values as low as 80, but the Hawke's Bay Regional Council's own measurements had found levels even lower.

"The Hawke's Bay Regional Council have done their own sampling around the Karamu catchment, and that's where they've found MCI values down to 60 which, as I said, I didn't realise MCI values could get that low."

He said students he had sent to the area had come back having seen dead animals on riverbanks and asked not to be sent to sample streams so badly affected again.

Photo: RNZ / Claire Eastham-Farrelly

If that kind of water made its way into a town's drinking water, it would only be a matter of time before people got sick as a result, he said.

"We have the highest level of many of these waterborne gastrointestinal diseases in the OECD."

Dr Death said he would not be surprised if a similar infection, particularly in high-intensity dairy areas, were to happen again.

"I think it's nearly universal around freshwater scientists at least that water quality in New Zealand has been declining for upwards of 25 years.

"There are a few measures in our waterways that seem to be improving slightly and many of the dairy industry put that down to their best practice management. It's questionable whether that's a result of that."

He said one key measure was nitrate, which came to waterways through cow urine, particularly when they had been fed palm kernel which they could not digest very well.

Photo: RNZ / Claire Eastham-Farrelly

"And associated with that urine, either from faeces as well or just because of where it's coming from in the animal, various other pathogens get carried into the waterways as well.

"In many ways, in the Havelock North area, I would be equally concerned about many of the chemicals that they use on their vineyards and orchards - none of those will be removed by any water treatment either."

Hawke's Bay 'leading the way' on water quality - Council chair

However, Hawke's Bay Regional Council chair Fenton Wilson told Checkpoint in a separate interview the council had already been making changes to lift MCI levels, decrease E coli and make the waterways a better place.

"In fact we're kind of leading the charge nationally."

"We certainly have set the bar higher, higher than in most parts of New Zealand. And we're implementing this process right now."

"There's a range of discussions throughout the plan change, it did go on for a couple of years.

"But all we're really interested now, John, is the outcome - which is a range of rules and regulations, including stock exclusion, land management focus on different soils, etcetera - a whole range of outcomes that will make the waterways a better place."

When challenged on whether waterways would improve soon, he simply said they had to.

"They have to get better soon, it's part of the process."