Why can't you complete the diploma out of the BComm contact hours...?
Unless I've misread something, you can (for BComm at least.) You just have to make sure that you begin the diploma at first year. This is made clear on page 13 of the PDF.
You can keep your core and your breadth and do a diploma of languages as well if you extend your degree by one year. But from 2010, you CANNOT commence the diploma of languages until 2nd year, so you reclaim your 6 breadth subjects from 2nd to 4th year at melbourne. Additionally, you cannot commence the diploma in 2nd year unless you have successfully completed 2 units of the language you intend to study.
For example - here is what I intend to do.
1st year: commerce subject, commerce subject, commerce subject, french (x2)
2nd year: commerce subject, commerce subject, creative writing, french (x2)
3rd year: commerce subject, commerce subject, creative writing, french (x2)
4th year: commerce subject, commerce subject, creative writing, french (x2)
So by the end of the 4th year I will have done 18 commerce subjects, 8 french subjects (diploma) and 6 creative writing subjects (which if you think about it is the same amount of breadth subjects you would get usually).
I hope this helps. This information can be confirmed on the website.