Do you want to do Japanese over other languages? If so, do Japanese. Languages require motivation, and if you are just seeking to become more 'employable' you will not reach a high standard. Weak motives don't last long in language learning. You may want to consider exchange / residencies, like what country you'd be okay living in for 3 weeks to 6 months.
If you are after $$$ and jobs, some would say Mandarin. Understandable, but Japan is still a close friend and ally to Australia, has a population of 125+ million, and wields a certain amount of power when it comes to business. It may not be the ~rising power~ China is, but when it comes to a diploma of languages and you are a first year, you do get to pick.
I suggest before you walk into your first Japanese class, you do some research on the language- even if its just learning basic phases. I'm doing Russian at Macquarie, and last year I was so unprepared. I've improved much better now, but a Diploma of Languages are a commitment. I wonder how UTS are at languages, esp Japanese.
You said you were doing business / law. What areas of business interest you? Do you see yourself having a global career? Want to practice law? If so, whereabouts in the world?