Although the CSSA trials often tend to specify, the actual HSC is very unlikely to do this. If they were to specify a certain speech, to be fair they would also have to specify a certain scene of King Lear, a certain extract from Cloudstreet, etc etc. It's too much work and would use too much paper. Also, they're not all that interested in the text itself but how you deal with it. Your skills and abilities, not their ideas.
So you can probably get away with not knowing all of the speeches really well. Definately learn the argument and some quotes from each one, as you don't know what question they might ask you and you want to be as prepared as possible. But choose a couple to learn REALLY well, probably ones that you can use for a variety of questions. (I choose the short ones (eg Lincoln) because they are easy to memorise entirely!)