With a US JD in Hong Kong, you are not pidgeholed into American firms. I spent years working for a UK based firm in Hong Kong. All the big firms in HK have significant US corporate (and in many cases dispute resolution/ competition/ compliance / trade, etc.) practices that have been around for a long time and are not going away.
UCLA has a huge network in HK. I know a couple people who went to UNSW and dozens who went to UCLA in HK - but that doesn't matter. The reputation of your school will have a small impact on your job prospects. So will the alumni network (I know tons of people who have talked to alums of their respective schools, very few who have actually been offered jobs by them). More important, for those employers who care, would be the rankings of actual degree programs and the university's overall ranking. But again, as long as the school ranks high, one vs. the other won't be a deciding factor.
You really need to have a specific goal in mind - what you want to do with your life. Then focus your education on that - go to a school and take the courses that best prepare you for that. If you really don't know, then the answer is that either school is probably fine since you don't have a very specific end game. Keep your grades way up wherever you go (having a high GPA cut off for interview is often the first way big employers with formal internship programs separate resumes and determine who will get a second look). Do internships and familiarize yourself with career services at your university- spend time at your career services centre at school wherever you are and get a sense of the types of jobs your alums have been offered recently (this is quite indicitave of what will be available to you) and also what internships are available.
Also, consider where you are actually from, and where you plan to be long term (if not Hong Kong). If the US, UCLA is the obvious choice. If Australia, UNSW makes more sense. If you don't land a job in HK right away or decide to work where you are from first, it will be easier if you studied there. Also, working in Aus or the US before coming to HK is something to consider. Company training programs are generally better
, offices are larger and there are more jobs available. Also you are exposed to a more 'mature' corporate environment before coming out here where things are HK-ish.