The ANZECC and ARMCANZ 2000 Guidelines for Fresh and Marine Water Quality define guidelines as follows:
"A water quality guideline is a recommended numerical value of an indicator (e.g. of a contaminant) or a descriptive statement (e.g. visual appearance of a water body) that will support and maintain the designated environmental values (EVs) of a particular water type. (Note that water quality guidelines are nowadays taken to encompass not only the physical and chemical characteristics of waters but also biological and even habitat characteristics)."
Guidelines are thus always related to a designated EV and are designed to provide a high level of protection to that EV. They are based on the best scientific information available at the time. There are a number of established guideline documents that apply to different EVs (more information).
Guidelines are often confused with water quality objectives. While guideline values are commonly used as de facto objectives, conceptually, the two are quite different. The definition of a water quality objective from the ANZECC 2000 Guidelines is:
"A water quality guideline was defined above as a numerical concentration limit or descriptive statement recommended for the support and maintenance of a designated water use. Water quality objectives take this a step further. They are the specific water quality targets agreed between stakeholders, or set by local jurisdictions, that become the indicators of management performance."
Thus, while guidelines are the technical basis of objectives, final water quality objectives take into account social and economic factors and are ultimately agreed to by all stakeholders.
As with guidelines, the term 'water quality objective' has traditionally been taken to refer only to the physical and chemical characteristics of waters. In modern usage, water quality objectives are taken to encompass a much broader range of characteristics including biota, habitat, flow and physical condition.