hi, i'm not 100% sure but i'll give you my opinion (emphasis on opinion) because i don't go to unimelb so i don't fully know how they do things.
basically i think that a grad diploma is something that you would have to consider in the final years of your degree because remember this is literally years away for you, things can change a lot in even one year, e.g. you may get lucky and get a journalism job straight out of your b.a or you may even decide not to do journalism.
from what i've seen and heard online, the media & comms major for the b.a at unimelb is quite broad so you won't only be focusing on journalism. i'm not sure how practical the major is but the broadness can be a benefit as it allows you to pursue other jobs in media if you decide not to do journalism.
i'm sure you will at some point learn practical skills since this is a practical discipline, and unimelb has a student-run newspaper i'm pretty sure so you can definitely develop some journalism skills there and it would be great to add to your resume when applying for jobs. i'm sure there's other student-run media like a radio, podcast, or whatever, and the uni offers internship subjects (but i'm not sure what their connections are for journalism). this would definitely assist you in gaining practical skills and it looks good when applying for jobs. you can also do a second major in something else like politics to increase your employability, as it's helpful for journalists to have a deeper knowledge of what they're focusing on.
i'm sure your parents have told you this but unimelb is basically the most prestigious university in our country and it has an excellent reputation on the global stage so i don't think your ability to get a journalism job would be significantly diminished by going there, even if it is more theory-based.
you should really sit down with your parents and have a long discussion about this. i'm sure that they want the best for you so if you really do prefer rmit you should try and let them hear you out so you can save yourself potentially one extra year of studying (if you do a grad diploma). creating a pros and cons list with them may really help as they may see that unimelb isn't the only pathway to a career. and also hear them out. yes they may come from different times but generally our parents are older and wiser than we are so sometimes they have valuable insights that wouldn't even come to our minds.
sorry for the long post, i went absolutely nuts researching unis last year when i was in yr 12 and i'm repeating the process again now that i'm looking to transfer into journalism. and please take this with a grain of salt because this is my opinion and i am not attending rmit or unimelb so i'm not an expert. i hope this helps and if you have anymore questions don't hesitate to ask!!