Hi Tina,
I was sort of in a similar situation. I didn't apply to NSW unis because I couldn't bring myself to live in Sydney, but I did apply to Melbourne and others. I thought it through before the offers / rejections came through and decided I would pay for Melbourne if it came to that. In the end, I got CSP at Melbourne, so it's moot now.
I intend to practice locally, though. I don't know what you mean by "getting into top international firms" means. As far as I know, you are limited to giving legal advice in the jurisdictions for which your registration is recognized. You can usually also give legal advice as an employee, but only to your employer. Ie. you won't be client facing. You can look into which jurisdictions provide reciprocal arrangements, and which ones need what, fairly easily.
You should consider that you will need to do your articles in the state you did your degree in. You will likely need to do this irrespective of where you intend to practice. The reciprocal arrangements are generally for people admitted as lawyers.
While there is a fair bit of money involved, you do not have to pay it back until you are working. Where you start out will have a dramatic effect on where you end up. I would pay it if you think it will help your career.
I can't tell you much about UNSW, but if you want to practice law in the US then it's a no-brainer. Melbourne is one of only three schools outside of US and Canada that is American Bar Association approved. Given two applicants that are from ABA-approved and not ABA-approved, you will need a very compelling reason for them to get past the ABA-approved guy. In the US, non-ABA-approved schools are a sick joke.
Whether the firm is US or not, they probably won't have heard of Melbourne or UNSW, so will look at the US News Rankings. Currently Melbourne is 36 and UNSW is 47. However, Melbourne is in a radical transition phase at the moment, so this will likely change. I can't tell you if it will change for the better -- though I suspect it will -- or not.
Owing to its dean, who has a background in international refugee law, Melbourne currently emphasises international law. However, he has recently resigned, so this may change by the time you get to electives. It also offers an arrangement to get "dual" degrees in China and the US whereby you get qualified in both jurisdictions. The US school is NYU, which is an insanely good law school. I am completely unfamiliar with Chinese law schools. I would expect competition to be fierce, so if you don't expect to do extremely well, I wouldn't give much weight to this.
As Frigid points out, they also have an arrangement for the BCL, but this is not the same thing as the dual degree programme. BCL does not confer any qualification to practice law anywhere. It's just a masters degree. You don't do it as part of your JD. All they have is a 5000 pound scholarship towards it for which you are eligible (but may not get) if you graduated from Melbourne law.
Hope this has helped.