The standard ATAR for usyd law is 99.5. Is the top 0.5% of HSC students.
Students can get in if they have a high wam, usually about 80-85, in a different first year course. However uni marks are not school marks. Typically only about 5 % of students at uni get a wam that high in first year
It's probably true that only about 5% of students have HD wams, but if you take into account that a student frequents BoS, has high standards, plans on getting into law, et cetera, the probability of that student getting a high enough wam in Arts (and presuming that they study something they're interesting in) to transfer into law is
much higher than 5%.
And plus, I'm only comparing to external transfers from law at other unis, which requires one to 1. get into law there, 2. do well consistently well enough to transfer into USYD law, and 3. stay on top of all the additional bureaucracy. Not everyone can get into it, but if a student wants to get into law at USYD, my personal recommendation is to internally transfer from within USYD.