slang - Why do Aussies use "cactus" to mean "dead," "useless," or "broken"?

I would like to suggest a different origin for 'cactus' - for several reasons. First, the slang term apparently originated in RAAF in the 1940s (according to the Wikipedia quote above), well after the introduction of Cactoblastis, and with no obvious connection to it. Second, in my experience the weed is always called 'prickly pear' in Australia, never 'cactus'. Thus I believe that 'cactus' is simply a euphemism for 'f*cked', chosen because of it sounds similar, but is innocuous (like 'darned' instead of 'damned'). During my apprenticeship in Sydney in the 1970's this was often spelled out, e.g. "It's cactus fuctus".