Napier Labour MP Stuart Nash is defending his claims about the health implications and efficacy of chlorinating public water supplies, despite disagreement from a panel of water experts at yesterday's Havelock North Water Inquiry.

Mr Nash's comments in a recent column in the Hawke's Bay Today were raised during deliberations about making treatment mandatory for New Zealand's drinking water.

In his piece, Mr Nash said a United Nations World Health Organisation report suggested there was an increased risk of bladder cancer associated with drinking chlorinated water, and that it was only proven to kill up to 60 per cent of known pathogens.

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He also promoted treating water with ozone as it killed 100 per cent of microbes that caused harm.

International drinking water expert and panel member Colin Fricker said the view that chlorine was linked to bladder cancer was declining, not increasing.

"There's been a report that suggests increased bladder cancer - that's a red herring - disinfectant by-products from a groundwater source are likely to be very low," Dr Fricker said.

"Chlorine is used in many places around the world - some use ozone at the front end but have chlorine at back-end - it's the No 1 primary and secondary disinfectant.

"I don't think those comments were appropriate - he said ozone was the way forward but ozone has a number of disinfectant by-products associated with increased kidney tumours."

Water quality scientist Dan Deere said he was not very familiar with the situation in Napier, but it was not the kind of source water that contained precursors to any cancer, and that overall he would be in favour of disinfection.

Water treatment specialist Iain Rabbitts said Mr Nash's comments were a "great example of the need to remove politics from water supplies.

"Chlorine is the biggest advance of the 20th century in terms of human life span - it's saved more lives than seatbelts and penicillin put together".

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Drinking water expert Chris Nokes said that if a supplier took substantial steps to ensure risks were being managed effectively chlorination could be avoided.

"But if you have a supply where E. coli is turning up something needs to be done and maybe chlorine is the easiest option."

Environmental scientist James Graham said that when E. coli results were present some form of treatment was needed.

He noted though that some communities were opposed to chlorination, including Maori communities.

"There are Maori cultural views that chlorination interferes with the wairua or spirit of that water source.

"It's important that those involved in water supplies respect those cultural views - mandating chlorination would be ignoring those views which would be unfortunate and probably discriminatory."

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Mr Nash stood by the research he had quoted and said that the E. coli readings found in Napier in recent months were a result of investment in the system to make it more robust, including changing the way monitoring was conducted.

He said that going down the route of chlorination was easy, and lazy.

"It's like putting a Band-aid on a sore, the sore is still there. I accept that yearly or twice a year maybe you flush the system with chlorine, but if they want to put chlorine in full time I will go hard against it.

"It's lazy - no-one has ever reportedly got sick from drinking Napier water. Money has to be spent on the infrastructure to get it right at the source."