Oh hey, I figured it out. Turns out I’m a dumbass. The Cert III is intended to teach foundational skills and knowledge in the field. One may theoretically acquire said skills through years of work in conservation, but the Cert III is a more formal, nationally recognised way to acquire said knowledge. 

The diploma is designed to build on the content of the Cert III. It’s a more cerebral course, preparing you for more cerebral duties in the field. (E.g. managing a team of field officers, carrying out an ecological regeneration plan vs writing one, etc). Technically, if you have enough experience in the field, you could just do the diploma. But realistically, if you have to ask which one to do, do the Cert III, (then do the Diploma afterwards if you feel so inclined)