 The term "automatism" describes unconscious, involuntary behaviour.  The legal rules governing the use of automatism evidence vary with the cause of the automatism, which may be disease of the mind, intoxication or some other circumstances.

 The use of evidence of automatism caused by intoxication is limited by the common-law intoxication rules and s33.1 of the Criminal Code. In particular, under s33.1, self-induced intoxication automatism is no defence to offences of violence

 Noninsane automatism may be produced by such causes as a blow to the head or physiological conditions such as poisoning, stroke or hypoglycaemia.  More controversial judicially recognized causes of automatism include some sleep disorders and psychological reactions to severe shocks - not including reactions to the ordinary stresses and disappointments of life.  These causes are distinguished from diseases of the mind by reference to factors such as whether the automatism could be attributed to an "external" cause, the ease with which the condition could be feigned, the likelihood of recurrence of the condition, whether treatment is required and medical or psychiatric evidence.

 For the defence of automatism to be used as a defence the loss of control must not be self induced, there must be complete loss of control and the cause of the automatism must be external such as a blow to the head causing concussion, the effect of a drug which may have been administered, hypnosis or post traumatic stress and there must be a total lack of awareness. For instance, should a swarm of bees enter a car causing the driver to loose control and hit a pedestrian the defence of automatism would apply.  Unlike other defences such as the special defences of provocation, diminished responsibility and suicide pact which only operate to reduce a charge of murder to manslaughter, the defence of automatism serves as a complete defence and is not limited. It is a defence to any crime including crimes of strict liability.

 The defence of non-insane automatism exists where a person commits a crime in circumstances where their actions can be said to be involuntary. This could be where for example an involuntary natural reaction occurs such as sneezing or being chased by a swarm of bees.  A finding of non-insane automatism may also exist where the defendant is not conscious of their actions due to an external factor often as a result of medication.  The essential difference between insane and non-insane automatism is that for insane automatism the defect of reason must be caused by an internal factor whereas for non-insane automatism the involuntary action must be caused by an external factor.

 Strict automatism is a denial of actus reus and therefore most commonly used as a defence against strict liability offences.  In cases of homicidal sleepwalking the illegal act is typically not denied but the intent to kill is.  Although one is not responsible for acts done whilst asleep, one can and is rightly held responsible for driving in a state where one would fall asleep at the wheel. The issue of prior fault applies to many diabetics who suffer hypoglycaemia whilst driving. Voluntary (and often involuntary) intoxication cannot cause legal automatism.

 If the accused committed the relevant act in a state of "sane automatism" he or she must be acquitted (R v Falconer (1990) 171 CLR 30; Hawkins v R (1994) 179 CLR 500).  This is because the accused cannot be convicted for an act which was independent of his or her will. The existence of a voluntary, willed act is an essential element of a crime (Ryan v R (1967) 121 CLR 205; R v Falconer (1990) 171 CLR 30; Hawkins v R (1994) 179 CLR 500).[12]

 John Dankweed was convicted of manslaughter after brutally beating his wife to death with a crowbar while she was asleep. Dankweed claimed non- insane automatism because he was sleepwalking during the incident.

 Sally Cackmunch was convicted of manslaughter after killing a pedestrian with her car. While driving, a swarm of bees appeared out of the back seat and blocked her vision, causing her to swerve and hit the pedestrian. She claimed Non-insane automatism because she did not have the mens rea to commit a crime.