Firstly, you can major in Actuarial Studies through the Bachelor of Commerce at the University of Melbourne (UoM). Another option is that Monash University has recently introduced this new major through their undergraduate Commerce degree. The only thing is that it is currently under review and hasn't been fully accredited yet.
At UoM, the degree is 3 years and if you have high grades, then you can do an honours year which extends your degree by one year. You have to also remember that to work as an actuary in Australia, you need to be accredited with the Institute of Actuaries of Australia (IAAust). It usually takes around 8 years to gain full accreditation.
For more information, I would suggest you visit
this page for more details. On that page, you can also download the Actuarial Studies brochure which will give you a lot more information.
Additionally, you can contact ILoveMathsMeth who is currently a first year Commerce student at UoM who intends to major in Actuarial Studies.