Mental disability in Australia has been difficult to define in legal terms. The reason is that if mental disability is defined in terms which are too narrow, then some disabled individuals will end up being denied the services they need. However, if mental disability is instead defined in terms which are too expansive then it may result in interventions which are unnecessary and detrimental. A general definition of mental illness actually refers to a range of emotional, cognitive and behavioral disorders which cause affected individuals to be less productive and interferes with their quality of life.
It is accurate to say that there is no single definition of mental illness which is currently agreed upon by all professions and legal jurisdictions. Those who attempt to formulate a working definition of mental illness for a practical application should take into account the clinical, legal and social definitions of mental illness and determine which definition, or combination thereof, is most applicable to the given situation.
In Australia, the standing legal definition of mental disability is laid out in the Mental Health Act 1990 (NSW), where mental illness is defined as any condition which presents symptoms such as hallucinations, delusions, serious thought disorder, severe mood disturbances and repeated behavior which is highly irrational and seriously impairs mental functioning.
Under this legal definition, a mentally ill person is someone who suffers from a condition which is severe enough to require some form of treatment, intervention or control in order to protect them from harming themselves or others. In making this determination, the present condition of the person is taken into account as well as the likely progress of their condition over time.