hsc is important if you want to get into certain courses quickly immediately
if you get a poor UAI you can get into courses through side doors, and then transfer to a different degree or a different university but this takes time. so if you want that degree in 4 years, hsc is important.
if you dont care if it takes you 5 or more years to get it, then you can bludge the hsc and try to get in the back way. but the thing is, getting in the back way means you have to work, so you can either do it now during the hsc or later on
so i guess its theoretically possible if you got a bad uai you could get into any random course with a low cutoff, work really hard and transfer your way all the way over to Usyd and do law.