Hey,
I have not done an actuarial major myself, but I have heard from people who are doing actuarial studies (Unimelb) usually do Specialist Maths, since it is a prerequisite for some maths subjects in first year (i.e. Calc 2).
I'm not sure if you're planning to do actuarial studies at Unimelb, but if you are, I'd also look at doing Further Maths, Accounting and Economics. This is because when you do commerce at Unimelb, first year economics, finance and accounting are compulsory subjects. I personally found the financial maths section of Further Maths somewhat helped me in understanding first year finance (I picked finance as an elective subject). I've also heard first year accounting covers a lot of the VCE accounting content again, and many people first struggle with accounting because they didn't do VCE accounting. Economics is doable for people who didn't do VCE, but I remember when I picked economics as an elective subject (and I didn't do VCE economics), I struggled massively. I've also heard people who did economics in high school found first year economics much easier to understand.
I'm not sure if this is the best advice, but I would recommend you to pick subjects you enjoy and would be most useful to your uni course. Uni is a big step up from high school, but I think if you have a good knowledge of some subjects beforehand, it will really help to provide a good foundation for your uni studies.