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4 Assessing compliance with the DWSNZ

4.1 Purpose

The drinking water that consumers receive should ideally be of consistently good quality. To achieve this, the water treatment plant must consistently produce water of good quality, and there must be no degradation in water quality as it passes through the distribution system from the treatment plant to the consumer. The purpose of assessing the compliance of a water supply is to determine whether these goals are being met.

4.2 Assessment

To attain overall supply compliance, the treatment plant and the distribution zone must comply. Furthermore, compliance must be achieved with respect to all contaminant types, ie, bacteria, protozoa and any chemicals, cyanotoxins or radiological contaminants that have been assigned as Priority 2 contaminants.

The NES is concerned with the ability of the treatment system to remove contaminants. For contaminants that are monitored at the treatment plant, the results of this monitoring will provide a good indication of how well they are being removed by treatment. For most chemical contaminants, however, compliance monitoring is undertaken in the distribution zone,14 so that these data too can be considered in assessing the efficacy of treatment. An important exception to the usefulness of distribution zone monitoring is monitoring for heavy metals.

Several heavy metals (antimony, cadmium, chromium, copper, lead and nickel) are almost always derived from the corrosion of household plumbing materials. In the past, monitoring of the levels of metals derived from corrosion was undertaken. This is no longer required, and these historical data provide no guidance on the efficacy of treatment processes. Although there are only a few instances, to date, of heavy metals being present in New Zealand’s source waters, new catchment activities could contaminate source waters with heavy metals.

4.2.1 Compliance monitoring

To determine whether water quality is being maintained over time, compliance is based on monitoring results collected over a 12-month period. Assessing compliance is therefore based on a series of samples – a single sample cannot be said to comply with the DWSNZ.

Water suppliers are responsible for taking these samples. The results of the sampling are evaluated each year, in collaboration with the drinking-water assessor, as part of the annual compliance assessment. The district health board saves a summary of these results in the Ministry of Health’s database Water Information New Zealand (WINZ) and they are transferred to Environmental Science and Research (ESR) for the preparation of the Annual Review of Drinking-water Quality in New Zealand which is publicly available ( (see also sections 11 and 12).

Bacteria and chemicals

The nature of a contaminant determines the way it is monitored. Contaminants that can be simply and cheaply measured are monitored directly, by collecting water samples and measuring the contaminant concentration. These contaminants include bacteria, chemicals and radiological contaminants. Cyanotoxins are also directly monitored, but their analysis is more expensive than the other contaminants listed.


For protozoa, sampling and analysis are more complex and expensive. For these organisms, performance parameters linked to treatment processes are measured. Performance parameters show whether treatment processes capable of removing the protozoa are operating satisfactorily, eg, the turbidity of the treated water or the intensity of an ultraviolet lamp. Performance parameters do not have MAVs. The equivalent benchmarks are operational requirement limits. These apply to the operation of treatment processes and are not associated with contaminants in the source or finished water.

Monitoring of performance parameters may also be used as the basis for bacterial compliance, if the chlorination process is continuously monitored and the water supplier wishes to take this approach.

4.2.2 Compliance criteria

Many factors, other than monitoring results, are taken into account when compliance with the DWSNZ is being assessed. These are not relevant to the NES as the assessments made for the NES will be based on MAV transgressions (see section 4.2.3) only. The additional factors are noted below, so that the reader is aware of what else might influence the ability of a water supply to comply with the DWSNZ.

A water supply (or a component of a supply, ie, treatment plant or distribution zone) is compliant when all of the following requirements are met:

  • the MAV of a contaminant, or the operational requirement limit for a treatment process, has not been exceeded (transgressed – see section 4.2.3 below) more than the permitted number of times

  • the contaminant concentration, or the performance parameter for the treatment process, has been measured at the required frequency (or more frequently) over the 12-month period

  • the monitoring procedures are correct (equipment has been calibrated as required; the correct test method has been used; and the analysis has been performed by a Ministry of Health-recognised laboratory)

  • the required actions to protect public health have been taken in the event of a transgression occurring, and steps have been taken to make recurrence of a transgression for the same reason, unlikely.

The details of these requirements are set out in the DWSNZ. Their discussion is beyond the scope of this guide, although more general points specific to the particular contaminants are discussed in later sections.

NES note: From the set of bulleted points above it can be seen that a treatment plant might be non-compliant, but not as a result of the concentration of a contaminant being too high. Consequently, if the effectiveness of a treatment plant in removing a contaminant is being assessed and the treatment plant is non-compliant, the reason for non-compliance must be determined to make an accurate assessment.

4.2.3 Transgressions

The term transgression is used in the DWSNZ to denote when an MAV or operational requirement limit has been exceeded. It refers to a single sample or event and may be either of two types, depending on which parameter is being exceeded:

  • an MAV transgression: the concentration of a contaminant is greater than its MAV

  • a performance transgression: a performance parameter is outside the operational requirement limits for longer than permitted.

A transgression is undesirable, but does not necessarily result in the treatment plant being non-compliant with the DWSNZ. Some transgressions are allowed provided enough other samples without transgressions have also been collected. The number of permitted transgressions before a treatment plant is non-compliant is given in Tables A1.3 and A1.4, in Appendix A1.8 of the DWSNZ. (These tables are reproduced in Appendix 1 of the present guide.)

14 Some chemical contaminants must be monitored in the distribution zone because their concentrations may change after treatment (disinfection by-products, for example). Chemicals that are not expected to change in concentration could be monitored at the treatment plant or in the distribution zone, but in most cases their monitoring is assigned to the distribution zone. This is more convenient if samples are already being taken in the distribution zone, and it provides a check on the concentrations to which consumers are actually being exposed.