Have you had a look around at all the graduate testimonials on the DFAT website? Fascinating insight into what courses people have done, and more interestingly, what previous work experience they brought with them to DFAT - as you'll see, it's really quite broad, you don't necessarily need to do International Studies, but anything with international relations would help given grad recruits without previous relevant study do a concurrent ANU course in international relations.
You may also notice that one of the grad recruits from 2005 was a B Economic & Social Sciences graduate from Sydney (!), and a Government major
(Gareth)
which is heartening for all aspiring Sydney Uni diplomats - which is practically every second person doing Government/Politics, and nearly all the kids doing B. International Studies. Speaking of this, as you probably know, it's really competitive, so keep an open mind, and at the end of the day, it's going to be a combination of grades, work experience and how you go in the selection process that'll have more say than specifically what degree you do.
And just to elaborate why B. International Studies should be approached with caution - a lot of the compulsory core units, especially in first year, are some of the most notoriously tedious and awfully taught units in the Faculty, namely Geopolitics, World Politics, and International Economy and Finance, and on the whole, it's a very structured, inflexible degree. You can practically do same things in a different degree with a lower cut-off (eg. B Econ. Soc. Sci), and the only thing you'll really miss are the 'practical' international studies units. Oh, and it also depends on what title you wanna graduate with, since they're inherently all the same anyway - 'Economic (& social sciences)' 'International studies', 'global studies', or even 'arts', depending on how you feel about the connotations of each title, and what title you want future employers to see.
Keep in mind that B. Comm (lib) is also a 4 year degree as opposed to just 3.