Deciding whether water quality is "good" or "bad" really depends on your point of view. It comes down to whether or not the quality of the water is suitable for the use the community wants to put it to, or the values it places on it. For example, the quality of Canterbury's waters determines their suitability for a variety of uses including drinking, mahinga kai, swimming and other forms of recreation, stock watering, industrial processing, and irrigation. The quality of water is also an essential factor for fish and other aquatic life, and is also a major part of the aesthetic and cultural values of waters.

For many uses and values, quality can be determined relative to certain minimum requirements, such as concentrations of contaminants. For example, water may be no longer suitable for drinking once the concentration of a particular substance in that water exceeds a particular value. When that happens the water is said to be polluted. But for other values, it is not so much the concentration of a contaminant that is important as where it comes from. An example of this is where the community places a high value on the "naturalness" of a water body. In this situation the presence of any contaminant caused by human activity may be unacceptable to some people.

But water "quality" is about more than just the presence or absence of substances in the water. For aquatic plants and animals, the water itself is only one part of a range of factors that impact on their habitat. Things like the state of stream bed and banks, depth and velocity of flow, and the nature and quality of adjacent vegetation are all important. When we think about water quality in its broadest sense we need to take into account all of these things.