I think a career as an actuary is pretty competitive regardless of where you go, and a degree both at Melbourne and ANU would be looked upon favourably (they are, after all, the only places where you can do actuarial studies). I guess the main question is where would you like to work afterward? I imagine with an ANU degree it would be easier to get a cadetship/job in the Canberra and Sydney area, and in Melbourne, you're more likely to get a job, well, in Melbourne. Maybe research the job market in these places in terms of type of firm and competition and pick whichever one appeals the most.
Also, with a 99.60, it wouldn't be unprecedented for you to get a later round scholarship with ANU, in which case I imagine both places would financially be fairly equal. The advantage of ANU would be the ability to do a double degree as well.