Related Rule
Australia
Practice Relating to Rule 72. Poison and Poisoned Weapons
Australia’s Commanders’ Guide (1994) states that the use of poison or poisoned weapons is prohibited. 
Australia, Law of Armed Conflict, Commanders’ Guide, Australian Defence Force Publication, Operations Series, ADFP 37 Supplement 1 – Interim Edition, 7 March 1994, § 932(b).
The Guide also provides: “Because of their potential to be indiscriminate in application, poison and poisoned weapons are prohibited.” 
Australia, Law of Armed Conflict, Commanders’ Guide, Australian Defence Force Publication, Operations Series, ADFP 37 Supplement 1 – Interim Edition, 7 March 1994, § 307; see also § 304.
The Guide further states: “The following examples constitute grave breaches or serious war crimes likely to warrant institution of criminal proceedings: … using certain unlawful weapons and ammunition such as poison.” 
Australia, Law of Armed Conflict, Commanders’ Guide, Australian Defence Force Publication, Operations Series, ADFP 37 Supplement 1 – Interim Edition, 7 March 1994, § 1305(p).
Australia’s Defence Force Manual (1994) states:
Poison or poisoned weapons are illegal because of their potential to be indiscriminate. So, for example, the poisoning or contamination of any source of drinking water is prohibited and the illegality is not cured by posting a notice that the water has been contaminated or poisoned. 
Australia, Manual on Law of Armed Conflict, Australian Defence Force Publication, Operations Series, ADFP 37 – Interim Edition, 1994, § 406.
The manual further states: “The following examples constitute grave breaches or serious war crimes likely to warrant institution of criminal proceedings: … using certain unlawful weapons and ammunition such as poison.” 
Australia, Manual on Law of Armed Conflict, Australian Defence Force Publication, Operations Series, ADFP 37 – Interim Edition, 1994, § 1315(p).
Australia’s LOAC Manual (2006) states:
Poison or poisoned weapons are illegal because of their potential to be indiscriminate. So, for example, the poisoning or contamination of any source of drinking water is prohibited and the illegality is not cured by posting a notice that the water has been so contaminated or poisoned. 
Australia, The Manual of the Law of Armed Conflict, Australian Defence Doctrine Publication 06.4, Australian Defence Headquarters, 11 May 2006, § 4.8.
The manual further states:
Provisions of the Hague Regulations 1907 are now recognised as part of customary law. Those regulations provide that the following acts are “especially forbidden”:
• to employ poison or poisoned weapons. 
Australia, The Manual of the Law of Armed Conflict, Australian Defence Doctrine Publication 06.4, Australian Defence Headquarters, 11 May 2006, § 13.29.
The LOAC Manual (2006) replaces both the Defence Force Manual (1994) and the Commanders’ Guide (1994).
Australia’s War Crimes Act (1945) considers “any war crime within the meaning of the instrument of appointment of the Board of Inquiry [set up to investigate war crimes committed by enemy subjects]” as a war crime, including poisoning of wells. 
Australia, War Crimes Act, 1945, Section 3.
Australia’s Criminal Code Act (1995), as amended to 2007, states with respect to serious war crimes that are committed in the course of an international armed conflict:
War crimeemploying poison or poisoned weapons
A person (the perpetrator) commits an offence if:
(a) the perpetrator employs a substance or employs a weapon that releases a substance as a result of its employment; and
(b) the substance is such that it causes death or serious damage to health in the ordinary course of events through its toxic properties; and
(c) the perpetrator’s conduct takes place in the context of, and is associated with, an international armed conflict.
Penalty: Imprisonment for 25 years. 
Australia, Criminal Code Act, 1995, as amended to 2007, Chapter 8, § 268.55, p. 336.
Australia’s ICC (Consequential Amendments) Act (2002) incorporates in the Criminal Code the war crimes defined in the 1998 ICC Statute, including “employing poison or poisoned weapons” in international armed conflicts. 
Australia, ICC (Consequential Amendments) Act, 2002, Schedule 1, § 268.55.
According to the Report on the Practice of Australia, the opinio juris of Australia supports the prohibition of poison or poisoned weapons. 
Report on the Practice of Australia, 1998, Chapter 3.2(3).