Related Rule
Australia
Practice Relating to Rule 20. Advance Warning
Australia’s Defence Force Manual (1994) states:
When a planned attack is likely to affect the civilian population, those making the attack are required to give, if practicable, effective advance warning of the attack to the authorities or civilian population. This requirement must obviously be applied in a commonsense manner in light of all other factors. If the proposed action is likely to be seriously compromised by a warning then there is no requirement to provide any warning. 
Australia, Manual on Law of Armed Conflict, Australian Defence Force Publication, Operations Series, ADFP 37 – Interim Edition, 1994, § 551; see also §§ 425, 733 and 924.
Australia’s LOAC Manual (2006) states:
When a planned attack is likely to affect the civilian population, those making the attack are required to give, if practicable, effective advance warning of the attack to the authorities or civilian population. This requirement must obviously be applied in a commonsense manner in light of all other factors. If the proposed action is likely to be seriously compromised by a warning then there is no requirement to provide any warning. 
Australia, The Manual of the Law of Armed Conflict, Australian Defence Doctrine Publication 06.4, Australian Defence Headquarters, 11 May 2006, § 5.56; see also §§ 7.36 and 9.25.
The LOAC Manual (2006) replaces both the Defence Force Manual (1994) and the Commanders’ Guide (1994).