Priority setting should be undertaken on the basis of a systematic assessment. The fundamental principle is that the assessment should be a collaborative effort. All relevant agencies should be involved, as much of the necessary information may be available in existing documentation from various sources, from previous studies or from a number of agencies. It may require the formation of a broad based inter-agency committee including authorities such as health, water resources, water supply, environment, agriculture and geological/mining to establish a mechanism for sharing information and reaching consensus on drinking-water quality issues.

Sources of information that should be considered to determine priorities include catchment type (protected, unprotected), geology, topography, agricultural land use, industrial activities, and sanitary surveys, records of previous monitoring, inspections, and local, and community knowledge. The wider the range of data sources used, the more useful the results of the process will be. In many situations, authorities or consumers may have already identified a number of drinking-water quality problems, particularly where they cause obvious health effects or aesthetic problems. These existing problems would normally be assigned a high priority.