Voluntary act of the accused
The question of whether there was a voluntary act of the accused that caused the harm to the victim which is the subject of the charge may involve one, or both, of two issues (see R v Katarzynski (2005) NSWCCA 72 at ):
Was there any act of the accused that caused the harm?
Was the act of the accused that caused the harm a voluntary one?
[4-355] An act of the accused causing the harm inflicted on the victim
This issue is dealt with generally under the topic “Causation” and the general direction given at [2-310] can be adapted where the issue is whether there was an act of the accused that caused the harm even though the particular act cannot be identified.
An issue can arise as to whether the act causing the injury was the act of the deceased or the act of the accused where the general directions on causation require considerable amendment. This is not a case where as discussed under causation, the issue is whether there was a break in the chain of causation by some act of the deceased or another person. But rather identifying whether the act causing death was the act of the deceased or the accused. For example, the issue can arise where the victim is given a substance by another person that results in the harm caused. In such a case the resolution of the question may depend upon the capacity of the victim to make a reasoned decision whether to ingest the substance knowing the consequences of doing so: see Justins v R  NSWCCA 242; (2010) 79 NSWLR 544.
In Burns v The Queen (2012) 246 CLR 334 at  it was held that act of ingesting drugs that were supplied by the accused to the deceased, was not the act of the accused. The ingestion of the drugs by the deceased was a voluntary and informed act of an adult.
[4-360] A voluntary act
The issue arises usually where the act causing death can be identified but the question is whether the act was voluntary. This can lead to a consideration of what should be considered to be the act causing death and is a question for the jury.
It is unnecessary for a trial judge to raise the issue of voluntariness with the jury if the evidence clearly suggests no lack of voluntariness: R v Whitfield  NSWCCA 501 at .
As to voluntariness see generally: Ryan v The Queen (1967) 121 CLR 205; Criminal Practice and Procedure NSW at [8-s 18.15]; Criminal Law (NSW) at [CLP.160].
As to the relevance of self-induced intoxication on voluntariness, see s 428G Crimes Act 1900 (NSW).
The particular issue of identifying the act causing death has arisen in homicide cases involving the use of a firearm: see Ryan v The Queen; Murray v The Queen (2002) 211 CLR 193; R v Katarzynski (2005) NSWCCA 72; Penza v R; Di Maria v R  NSWCCA 21 at  but see also Ugle v The Queen (2002) 211 CLR 171 and R v Whitfield. The issue in the firearm cases is whether the involuntary discharge of a weapon can be seen as the act causing death in the light of all the evidence surrounding the production and discharge of the weapon. This is a question for the jury.
It is difficult to set out a suggested direction because what needs to be said to the jury will depend upon the particular facts. But the direction should include the following general statements.
[4-365] Suggested direction — voluntary act
The act causing [the harm] must be the deliberate act of a person before that person can be held criminally responsible for the consequences of that act. An act is not deliberate if it was not voluntary. To give rise to criminal responsibility the act must be a willed act of the person accused of committing an offence. A spontaneous, unintended reflex action is not itself a voluntary act. In common speech a person will describe an involuntary act as being an accidental one. The Crown must prove beyond reasonable doubt that any act of [the accused] upon which it relies as causing [the harm] inflicted to [the victim/deceased] was a voluntary act: that is, a willed act on the part of the accused. This is distinct from the issue of whether the accused intended certain consequences from his or her act. It is a more fundamental concept that is concerned with the nature of the act itself.
Here [the accused] has raised the issue of whether [his/her] act resulting in [the harm] to [the victim] was a voluntary one.
[Indicate the basis upon which it is asserted the act was not voluntary and the evidence in support.]
The Crown must prove beyond reasonable doubt that the act alleged as causing [the harm] to the [the victim] was a voluntary act of [the accused]. If you consider that the Crown has failed to eliminate the reasonable possibility that the act [of the accused] relied upon by the Crown was not a voluntary one, you must find [the accused] not guilty.
[If the issue of what act of the accused caused the harm arises see the suggested direction for causation at [2-310].]