Related Rule
Australia
Practice Relating to Rule 19. Control during the Execution of Attacks
Australia’s Commanders’ Guide (1994) provides: “An attack must be cancelled or suspended if it becomes apparent that the target is not a legitimate military objective and excessive collateral damage would occur in relation to the direct military advantage.” 
Australia, Law of Armed Conflict, Commanders’ Guide, Australian Defence Force Publication, Operations Series, ADFP 37 Supplement 1 – Interim Edition, 7 March 1994, § 957(d).
Australia’s Defence Force Manual (1994) provides an example of the obligation to cancel an attack when the object is not a military objective or is subject to special protection:
For example, aircrew may be ordered to bomb what the mission planner believes to be a command and control centre. If, in the course of the mission, the command and control centre is displaying an unbriefed symbol of protection, eg Red Cross symbol, then aircrew must refrain from completing their attack. The Red Cross symbol indicates the facility is a protected installation and is immune from attack unless intelligence, or higher authority, determines that the facility has lost its protected status because the emblem is being misused. 
Australia, Manual on Law of Armed Conflict, Australian Defence Force Publication, Operations Series, ADFP 37 – Interim Edition, 1994, § 832.
Australia’s LOAC Manual (2006) states that, during the execution of attacks, it is the commander’s responsibility to
cancel or suspend the attack should it become apparent that the assessment made under dot points 2 [assess the target as a valid military objective and otherwise unprotected by the LOAC] and 4 [assess collateral damage as proportional] are no longer valid. 
Australia, The Manual of the Law of Armed Conflict, Australian Defence Doctrine Publication 06.4, Australian Defence Headquarters, 11 May 2006, § 5.62.
In its chapter on “Air Operations”, the manual states:
8.35 An attack must be cancelled or suspended at any stage where it becomes apparent that the objective is not a military one, is subject to special protection, or the attack may be expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof, which would be excessive in relation to the concrete and direct military advantage anticipated.
8.36 … For example, aircrew have a special and personal responsibility, under the Additional Protocols, to ensure LOAC compliance if they acquire information that was not available at the planning stage. For example, aircrew may be ordered to bomb what the mission planner believes to be a command and control centre. If, in the course of the mission, the command and control centre is displaying an unbriefed symbol of protection, eg Red Cross symbol, then aircrew must refrain from completing their attack. The Red Cross symbol indicates the facility is a protected installation and is immune from attack unless intelligence, or higher authority, determines that the facility has lost its protected status because the emblem is being misused. 
Australia, The Manual of the Law of Armed Conflict, Australian Defence Doctrine Publication 06.4, Australian Defence Headquarters, 11 May 2006, §§ 8.35–8.36.
The LOAC Manual (2006) replaces both the Defence Force Manual (1994) and the Commanders’ Guide (1994).