It makes a lot of sense to me. You want your primary public facing site with a TLD that people know about. .edu immediately confers legitimacy by assuring the visitor that its a legitimate educational institution (I’m assuming .edu.au is the Australian equivalent).
But when it comes to sites and services students and professors would want to use, go for the TLD, (e.g.s alumni.rmit, bursar.rmit, athletics.rmit). It’s a much more pleasing (I don’t know whether rmit actually does that or not).
Interesting that the university's website is not on that TLD but rather https://rmit.edu.au