A public health expert says he hopes a report showing some drinking water doesn't meet safety standards won't deter people from drinking tap water.

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The Ministry of Health report says 19 percent of the population is exposed to water that doesn't meet all the safety standards.

Michael Baker, who's a professor of public health at Otago University, said the water coming out of the taps was generally very safe to drink.

"I think it would be a real mistake if New Zealanders were turned off tap water as their main source of hydration and it would be I think a disaster if we switched to drinking bottled water as you have to do in some countries, because obviously that has huge negative environmental impacts, and it's also very expensive."

Mr Baker said people on smaller supply systems had more cause for concern.

"The 20 percent of the population who are not covered by this monitoring system and they are generally people on supplies that service less than 100 households, and these are the places that have most of the outbreaks and generally if you look at the smaller supplies in the monitoring system that's where most of the problems are."

However, around 96 percent of reticulated water supplies met the bacteriological standards, which meant they provided reasonable protections against things like campylobacter and other serious pathogens, he said.

But only around 83 percent of these water supplies meet what is known as protozoan standards, which gives them the ability to remove things like giardia and crypto, consequently they don't have all of the features needed to protect the public from those pathogens.

Climate change and intensification of agriculture were two major pressures on drinking water systems and the more general environment, he said. That was illustrated by the Havelock North campylobacter outbreak in Hawke's Bay which occurred after sheep faeces were washed into the aquifer and contaminated the drinking water supply and this outbreak happened after heavy rain.

But Mr Baker said it was unlikely to happen on this scale again because of an almost universal disinfection of water supplies using chlorination, which minimises the chance of this type of event.