I actually thought 75+ WAM was hard for first year before I started uni, but not too hard actually (you seem smart).
I got a bit more, but kinda got carried by the math courses (should get HD's and definitely since you're doing adv. maths).
with effort, 60 wam should be a walk in the park, 70 wam requires consistent study/being up to date (only 1-2 week behind max) and 75+ requires alot more diligence and just all-round consistency.
the effort i put in should only net me around 65 wam, but im able to understand things really quickly so i guess that carries me
You'll need to go through hell to maintain an HD WAM. I know a few people who did and damn that was hard. No joking around or surfing net at all during lectures/tutes and always doing prep. Not easy unless you don't have other commitments.