Firstly, are you sure you actually interested in investment banking? There are SO many people out there who see the big $$ that IB offers and suddenly they have their heart set on a job that they actually don't know anything about. If working 90 hours a week is unappealing to you, then don't bother.
As for your question, I can't see how actuarial science will be of benefit to you in anyway that a standard commerce major like Finance wouldn't be. From what I have researched, by the far the most common qualifications to enter IB is Commerce/Law, which is followed by Commerce with honours. I don't think IB firms care a huge deal about the specific degree you have, but rather that you can demonstrate a strong interest and aptitude with finance and that you are not socially retarded. That being said, if you can do really well in actuarial science then I'm sure that will impress recruiters as it is notoriously difficult.
Keep your options open though. IB is extremely competitive to enter, and it is quite a small field in Australia (relative to say, New York City or London).