Well, I'm a postgrad student in maths with interests in mathematical biology, which is pretty close to what you're after. Here's my take on it: do either Applied Maths or Statistics and Stochastic Processes, depending on whether you're more excited by calculus/physics or statistics. The DM/OR major is more applicable to business than to science. The Statistics major has the added bonus that you become eligible for the GStat (i.e. Graduate Statistician) qualification from the Statistical Society of Australia (their requirement is 4x third year + 2x second year stats subjects). The Applied Maths major has a core unit on computational methods, which could be worthwhile if you haven't done any other computer science.
Looking at specific units on offer: the third year units MAST30030 Applied Mathematical Modelling and MAST30001 Stochastic Modelling cover the techniques that are likely to be most useful to you. From what I can tell, you can do these subjects as part of either major mentioned above.
I'm not sure how far through your course you are already, but if you're uncertain which way you want to go, you should choose the first and second year units that let you pursue either major.