First year advanced maths courses pretty are similar to the mainstream courses, except has slightly more content (e.g. a more rigorous treatment of limits), and offers more challenging problems. However once you reach second and third year, a lot of the advanced courses are completely unique (i.e. there isn't a normal stream counterpart).
(A few second and third year advanced maths courses put you into the mainstream lectures [e.g. financial maths, mathematical computing, statistics...], but offer additional problems/content/lecture, but there aren't very many of those. Popular advanced maths courses are very much separate or entirely unique.)
I just finished first year Advanced Mathematics, and doing relatively well (HD average in advanced maths courses in semester 1, and probably higher this semester). In my HSC, I only got 93 reported mark for Extension 2. As long as you apply effort to it, advanced mathematics is perfectly doable, and very enjoyable. If you are interested in mathematics, I strongly encourage you to try it out. Even if you feel like that the workload is more than you are comfortable with once you've started uni, you can always drop down to mainstream mathematics in a standard Bachelor of Science, which is still a great degree per se.
Just a reminder that you need to apply for Bachelor of Engineering and Bachelor of Science, then apply for a stream transfer into Bachelor of Science (Advanced Mathematics) after getting into the course. Alternatively, you can study advanced maths courses under the normal Bachelor of Science degree, then transfer into Advanced Mathematics later if you getting distinction average (which isn't too difficult honestly as long as you try).
Good luck!
