Hey,
I got offers to do combined actuarial @ mq and unsw and ended going up with mq, but I didn't get the required atar for either. I only got an atar of 96.5, but I got 5 bonus points from mq and 2 from unsw...
Anyways you can and you cant, answering your questions above. To become a qualified actuary in Australia or a fellow of the institute of actuaries australia (fiaa) you need to complete part 1, part 2, part 3 exams and a professionalism course. Depending on your course/grades, you can gain part exemptions to part 1 and 2 (just means you dont have to pay the institute to do it) in a combined undergraduate degree or a postgraduate, which is typically a master of actuarial practice.
However if you dont get the 97-98 atar you still need an atar >95 and have completed a maths or stats undergrad degre for the institute to allow you to become a student member and sit your exams.
http://www.actuaries.asn.au/becoming-an-actuary/becoming-a-member/becoming-a-student-member
University Graduates
Graduates from an Australian or New Zealand university should have a degree majoring in mathematics or statistics, or honours in a non-mathematical subject, and demonstrate a sufficient standard of mathematics through university or school grades.
Graduates need to have completed their Higher School Certificate equivalent and received a ranking of 95 or higher when converted. Subjects must include English and high-level Mathematics.
All hope is not lost though. You can definitely transfer from another relevant stem degree part way during uni but again it's hard. In all honesty just do your best and see what happens.
Btw actuarial @ anu is only 95 atm. So yeah i would say if you get >98 awesome ur in, 97-98 probs, >95 maybe, >90 unlikely but best of luck!
Also I think UNSW has a future students day where you get to go into the city and have a mini lecture/tour of Finity's offices. Finity are one of the leading actuarial consultancy firms in Aus/NZ and I went when I was in y11 and it was super cool. Could be something there. I think MQ also has a similiar be an actuary for a day for y10-12 students...