hey there,
hopefully i can help you out a bit, im also a bsci student doing a diploma (italian) and going on exchange next year (during my third year).
Firstly full time load means atleast 3 unimelb subjects (12.5 * 3 credit points), so as an exchange student you have to do an equivalent to atleast 3 (which I would recommend, as you want to enjoy your time on exchange and not spend the whole time studying plus overseas institutions can often have more rigorous courses with no recordings
I'm not 100% sure what you mean by run out of subjects. Do do a bsci and dip you need to complete 400 points so if you did 4 each semester you will have done 100 points, (probably a mix of credits between you bsci and dip). You won't be able to complete the rest of your diploma subjects while you're overseas, I've also heard that some japanese universities don't offer actual japanese classes (i.e for foreigners) so you should double check that you can actually study japanese or japanese culture subjects for foreigners that will be equivalent to classes in the diploma.
To find out the specifics of particular subjects, such as comp sci, you would need to find the exchange uni's equivalent to our handbook and have a read through about the subject.
What you need to understand, which is not well explained unless you go to stop 1, is apart from core/prerequisite subjects for your major which need to be extremely close equivalents in order to waive the prerequisite, the other subjects you study can literally be about anything! You will come back to aus, and instead of saying you completed e.g chemistry 2, itll just say you got 12.5 credit points (if that makes sense)
If you're seriously considering exchange, try your hardest in first year to do as many core/prerequisite subjects (even some level 2s if possible) so that you can study pretty much anything on exchange.
hope it helps, but honestly don't stress about it now, you will have all of first semester in first year to figure this out and unlimited time to ask stop 1 so don't stress.