Overall though, I expect that maths units are focussed on different things, e.g. more advanced calculus, trig etc... (someone doing maths/sci/eng feel free to correct me here/clarify this). Doing maths units would probably make finance units feel rather easy, but I'm not sure they'd be very complementary or beneficial.
So, high school maths is a pretty crap introduction to what maths is really all about (albeit it does teach some essential skills). See, there's three main variations of maths:
Pure MAY be helpful, but it probably won't be obvious in how it's helpful. Applied may also be useful, but I doubt you'll need extremely tricky applied stuff for economics. Stats, however, will definitely be useful - if not now, then later on in life.
http://monash.edu/pubs/handbooks/aos/mathematics-and-statistics/ - this is all the maths subjects offered by Monash. If the unit has the code MTHx2xx, then it's a stats unit. Feel free to peruse and see if there's anything you like. Note, though: to do most stats units, you need to do MTH1030 and MTH2010/MTH2021 (I'd suggest MTH2010 myself, because I think it pops up more).
If you look through the maths subjects, you also notice that a lot of the stats units either have prohibited units with eco, or you can do them with eco background knowledge and little maths background. Note, though: while there are applications of stats in business and economics, maths units ARE maths units - they're going to be a lot more intensive than any non-maths unit in terms of the maths being done, and there'll be less focus on the business side of things.
You should probably ask TrueTears if you really want to know, but here's my perspective as a maths lover:
MTH2222 and MTH2232 are both very useful for a background knowledge in stats. If you want to see more applications rather than cool maths things, MTH3241 and MTH3251 will both cover applications in economics (MTH3241 only chucks it in at the end, but it's there, Kais loves his financial modelling) and you can get in with just ETC units.