What the RSPCA told me.
Hopstop appears to be sufficiently humane to be recommended by the RSPCA. Hopstop contains an amphibian anaesthetic along with its lethal ingredient. Cane toads display some signs of distress after being sprayed (trying to bur…row into the ground, limb movements and urination) but not more severe ones such as toxin secretion and gastric eversion. Also, following spraying the heart rate steadily slows in comparison to an increase following some other treatments such as Dettol. The toads become unconscious within 2-5 minutes and die within the next 45 minutes without gaining consciousness.
Hopstop is easy and practical to use. The instructions must be followed and the cane toads disposed of following their death. The spray will kill other amphibians and fish if used indiscriminately.
The other method RSPCA advocates is Stepped Hypothermia (basically placing the toad carefully in a plastic bag, and putting in refrigerator for 12 hours at 4 degrees – this anaesthetises it. Then the toad can be placed in the freezer for at least 24 hours – this will painlessly kill the toad. We then advocated burying the body deep enough that other animals can’t dig it up.)