I don't see how that argument is valid. If you're an employer, are you going to be more interested in the university they went to (because it might be an indication of their intellectual capacity), or the marks they have achieved in the field they are applying for during university? I believe the latter is a far better representation of an individual's ability.
I will not say whether UWS graduates are disadvantaged when applying for jobs in top tier law firms, simply because I do not know. I am not a law student (yet), and I doubt employers would openly admit that they are prejudice. In saying that, I do believe that if you have good marks, involve yourself in mooting competitions, work experience, employment and present yourself well in an interview, the influence that the university you attended has over your ability to gain employment should be at least diminshed somewhat.