Hope this is not too late to reply:
I sat the MST two years ago, and found it not terribly hard, bearing in mind that I never did AMEB and all my theory came from high school + HSC music 2. Am trying to remember vaguely what it contained... I seem to recall some general knowledge questions, and the last part was a choice between harmonising a melody with a set of chords or writing a four-part chorale. I was surprised at how easy the aural dictation section was compared to the music 2 exam.
Basically, the test is used to make sure that a) you can actually read and understand music, and b) you are put in the right stream for music theory if you get offered a place. My heart still stings a little when I remember finding out that I got put in Harmony F.
Hope that helps!