Yes, the major is started first year. However, the subjects in first year are not actuarial subjects (only Intro to Actuarial Studies is) and cover a wide range of commerce areas (micro, macro, accounting, maths etc.) and you get one elective/breadth subject in each of the 3 years.
Thus, if you find that after Intro to Actuarial Studies, that actuarial studies is too hard/not for you/boring/etc. you can easily pick up, say, a finance major and continue for the next two years. If everything's fine, then you can move onto second, third, honours years etc.
The BCom (majoring in Actuarial Studies) with an honours year at Melbourne teaches you the course for Part I and II of the IAA actuarial exams - and if you perform well in your uni subject exams, you can get exemptions from these.
http://www.actuaries.asn.au/ for more details. After Part I and II, you pretty much become an associate of the IAA>> getting a fellowship requires you to do a course through the IAA and more exams.
Hope that is of some help. The course is, I've heard, hard but not impossible if you really enjoy it and try hard. Don't be so negative!
And you can always transfer to another major after first year if you don't like it. Are you interested in any other areas? It's great to have alternatives - I really like law and eco and maths too so I will be getting those subjects too... I might end up at uni for 9 years if all goes to plan... which it probably won't

PS: PM'ed you with more information about actuarial studies at Melb. Good luck
