Complacency, inept officials – a Government inquiry paints a frightening picture of the state of New Zealand's drinking water, with at least 750,000 of us drinking from supplies that are "not demonstrably safe" - a figure described as likely to be a "significant underestimate".

The inquiry was sparked by the 2016 Havelock North gastro outbreak, which has now been linked to four deaths, and calls for a major overhaul of water supplies, including mandatory treatment.

The Government has now written urgently to all mayors and district health boards asking to check the water they are supplying meets current standards after the inquiry revealed 20 per cent of water supplies were not up to standard.

inthehouseNZ/YouTube David Parker makes his Ministerial Statement about the report in Parliament.

That 20 per cent affects 759,000 people, of which 92,000 are at risk of bacterial infection, 681,000 of protozoal infection and 59,000 at risk from the long term effects of exposure to chemicals through their water supply.

But that figure was likely to understate the problem, as it did not include more than 600,000 people who drink water from self-suppliers or temporary suppliers, or tourists to places like Punakaiki on the West Coast, which is under a permanent "boil water" notice.

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MARTY SHARPE/STUFF Mangateretere Stream, which is near one of three bores that supplied Havelock North's water, was identified as a likely source of contamination. Sheep faeces from a nearby paddock likely entered the stream.

The inquiry found that complacency about the state of New Zealand's drinking water was common, yet the evidence showed that in many cases it was safer to drink tap water overseas than here.

But its most damning findings related to the Ministry of Health, which it described as inept and negligent in its oversight of a system in which non compliance with safe standards was high.

The risks for contamination of the water supplies were detailed by the inquiry including damaged pipes, a huge number of private and unknown bores, and the close proximity of sewerage to drinking water assets, a factor that caused surprise among overseas experts.

Other risks were earthquake damage, climate change, disused landfill sites – as many as 1000 – and deteriorating infrastructure, a recent example being collapsed asbestos cement pipes in Marton, affecting drinking water supplies. An estimated 9000 kilometres of similar pipelines are in New Zealand. Many were reaching the end of their lifespan and the cost of replacement was estimated to be about $2.2 billion.

In its report, the inquiry noted the high number of times communities had to boil their water – "in 2015-2016, 44 supply zones had boil water notices issued affecting 15,000 people. Twenty-six of the boil water notices were permanently in place (affecting 7200 people).

"Following the Christchurch earthquakes, 281,000 people were affected by boil water notices, of which 9300 were on a permanent boil water notice. These statistics do not of course take into account the fact that many other people, including tourists, are impacted by the boil water notices."

SUPPLIED Tens of thousands of kiwis at risk from contaminated water.

Internationally we do not compare well – "public supplies in England and Wales, large Finnish supplies, and Scottish Water have all had greater than 99.8 per cent compliance with E coli standards from 2011-2015," the report found.

"Moreover, in England and Wales there have been virtually no issues with protozoa during the same period. Compliance rates for smaller, private or community supplies in the above countries do remain significantly lower."

It was estimated there were currently about 35,000 cases of acute gastrointestinal illness contracted from reticulated drinking water per year.

The inquiry followed a deadly gastro outbreak in Havelock North in August 2016, and was carried out in two stages, the first being to report on the causes of that contamination.

The second part of the inquiry looked at broader water quality issues.

It found that lessons from Havelock North appeared not to have been learned – compliance figures in the 2016-17 period were still "alarmingly low" and "do not appear to reflect any increased vigilance by suppliers in the aftermath of [that] outbreak".

"The inquiry found the falling compliance levels with the bacteriological and chemical standards particularly concerning. The decrease in compliance with the bacteriological standards results from an increased number of transgressions, an increased number of supplies with ineffective, delayed or unknown remedial action following transgressions, and an increased number of supplies with inadequate monitoring.

"Twenty-seven supplies failed entirely to take any remedial action after a transgression. In the aftermath of the bacteriological outbreak in Havelock North, these failures to respond effectively to transgressions or to monitor adequately are surprising and unacceptable."

Attorney General David Parker sheeted some of the blame home to the former Government, saying Labour had inherited a "mess" from National.

"One of the things the report makes clear is that central government has for at least five-years known that water supplies have not been doing their duty and the Ministry of Health and those responsible for them have effectively failed New Zealanders.

"They say that over the last five-years not one compliance notice, let alone a prosecution was brought, not withstanding the fact that they knew that we had serious problems in some parts of New Zealand and it wasn't getting better," Parker said.

But Parker, and Health Minister David Clark, stressed that 80 per cent of water supplies - largely Auckland and Wellington - were up to standard.

* An earlier version of this story said 1 million Kiwis were exposed to potentially unsafe drinking water, the report specifies 759,000 affected people, but describes that figure as likely to be a significant underestimate.