if u want exposure to more maths:
since u have so much time there is alot more you can do. I would recommend entering maths competitions, particularly the Australian maths competition and unsw school maths competition in addition to any others (like icas). Also would strongly recommend BoS trials. You also have the option to read maths journals or whatnot, or maths Olympiad books which are extremelyyyyyyyyy difficult, where each question is meant to take about an hour or more. Also maths YouTube videos:
-https://www.youtube.com/channel/UCjwOWaOX-c-NeLnj_YGiNEg
-https://www.youtube.com/channel/UC1_uAIS3r8Vu6JjXWvastJg
-https://www.youtube.com/channel/UCEI9wNw9a4cJfejeAU6J2wQ
However textbooks are better, although vids aren't a bad start.
good genres of textbooks focused on problem solving:
-Olympiad textbooks (very advanced.)
-maths (pre-university)puzzle textbooks (fun+ mentally stimulating with a focus on creative thinking usually. in particular Professor Povey's Perplexing Problems)
-textbooks focused on a specific topic, with an emphasis on proofs. (you will likely see many potential q16s)
I focused somewhat on extracurricular maths because i found it funner. Although i feel it has helped, it should not make up the bulk of your study.
With enough exposure+ doing past papers you should eventually get better.
However I can't accurately comment because I didn't get 99, but doing the above will hopefully build you problem solving abilities, and knowledge in the area.
https://www.matrix.edu.au/matthew-winfreds-hacks-for-extension-2-mathematics/
this article says to focus on building a deep understanding of all the content which is probably the best approach tbh. I would recommend following it in particular on your last year of high-school.
I would also recommend not looking at the solutions, but spending alot more time trying to answer every question. Solving a difficult problem that you couldn't solve initially will be worth the effort in the long run, since problems of that nature will become easier to solve later on(moreso than reading solutions). I would argue that this is the best way to improve. Perhaps keep a list of questions you found difficult and do them later on. ekman has a good list of problems.
Since you have so much time, you can try many different things and reflect on what feels effective, especially since you have a way to track your progress (past papers). However studying for exams should come first(although maybe not in prelim), the article above dictates a pretty good approach.
But that being said there is alot more to do then just getting better at maths. For example you could enter some university computing courses in high school(
https://www.engineering.unsw.edu.au.../school-programs/high-school-computing-hs1511 ). It assumes no prior knowledge btw.
Knowing the content 2 years ahead of time is a really big advantage. If study regularly for it, you will almost certainly get a great mark.