Hey, SUPER good question.
I definitely don't have any official answers, but, I'd imagine if you achieved a 70 average at USyd, UniMelb would consider that as a 'D' (or H2B, I think, in their system). Amongst Group of 8 Unis, I doubt that what the mark is called is the important thing to them. I'd wager they'd care more about the mark itself.
I can't speak for whether or not it's harder to get an HD at a NSW uni based on the grading system. Perhaps the tutors adjust their marking accordingly and think "This essay deserves an HD" and awards an 86 accordingly, whereas a VIC tutor moght think "this deserves an HD" and award an 81. If the marking is similar, though, which I'd imagine it would be... Then absolutely harder to get an HD in NSW haha.
There are probably people who coudl give a way better insight than me but - as far as university transfers go - I think you're safe.