I thought describing how the supply of drinking water is regulated in New Zealand would be a doddle. Little did I know. The picture below describes some of the entities and relationships involved but it is not comprehensive.
The Legal Environment
From the earliest days of local government until 2008 it was up to each council to provide safe drinking water as it saw fit. Local Medical Officers of Health always had powers to intervene if a water supply got into serious problems but councils were generally only ultimately answerable to themselves.
That changed in 2008 when the Health Act 1956 was changed to make it mandatory for drinking water suppliers to register themselves, comply with Drinking Water Standards, and get a Water Safety Plan approved. The Act also creates offences with stiff penalties for anyone who didn't comply.
The Ministry of Health is the top dog. It administers the Health Act and the Minister of Health issues the Drinking Water Standards. It appoints the Drinking Water Assessors who work regionally out of a DHB. These assessors have the job of maintaining the register of water suppliers and approving Water Safety Plans. The Medical Officer of Health (also working out of the DHB) has emergency powers in the case of a major public health incident. The MoH contracts out a lot of the technical work to ESR who seem to run a semi-autonomous operation at drinkingwater.org.nz.
The Ministry for the Environment do a little "me-too" work in this area advising regional councils on how to comply with the NES Sources of Human Drinking Water. But they are not really a player.
Drinking Water Standards
These standards have been around in various versions since 1993. They mainly specify what levels of contaminants are acceptable in drinking water. There are some engineering standards dotted around - most significantly a brief requirement for securing well-heads.
As part of the pre-2008 voluntary regime water suppliers could seek a grading on their individual supplies. The grading was based on a combination of design of the supply system and track record (shown by the results of regular water testing). Hastings District Council at one point decided to abandon the voluntary grading in favour of concentrating on the statutory Water Safety Plan. A quick look at the database of water suppliers suggests to me that many other councils are also opting out of the grading system or putting a low priority on seeking a grading.
Water Safety Plan
If the Drinking Water Standards are basically the performance targets to meet the Water Safety Plan is basically the "how-to". It is part description of the supply, part operations manual (especially testing), part contingency plan, part plans for the future.
The key thing to remember is that it is a risk management plan. It may or may not arise in the Inquiry but the point is the whole regulatory regime is designed to minimise the risks of a contamination event not to prevent them.
The elephant in the room
I would be surprised if this got an airing in the Inquiry but one of the biggest questions for me is how come the worst public health incident we have had in living memory came after the government takeover?
Prior to 2008 the public health units at DHB's kept a very close eye on major water suppliers but, legally, the relationship was voluntary. The mechanism for nudging councils towards higher standards was the league table of grades assigned to each water supply.
The amendments to the Health Act put the government unequivocally in charge. All we have got for that so far is the worst water-borne illness incident ever.