The Environment Protection Act 1970 defines a "septic tank system" as a system for the bacterial, biological, chemical or physical treatment of sewage, and includes all tanks, beds, sewers, drains, pipes, fittings, appliances and land used in connection with the system. Therefore, waste water treatment plants and composting toilets are types of septic systems.
A septic tank system treats and retains sewage (toilet water) and sullage (bath, shower and sink water etc) within the boundaries of your property.
The Septic Tank Installation and Maintenance Guide contains important information about installing or altering a septic tank system.