Can I just point out that comparing us to South-East Asia is a really bad idea, considering that that entire region is very maths/science dominated? Seriously, Japan doesn't even have Arts/Humanities degrees per se - the closest things on offer are "Social Science" programs, and even in those you have to do some science, from what I hear.
I'm also wondering about the cause behind the number of maths students; I'm interested as to whether or not it has something to do with the ranking-based system of the VCE (meaning less students are willing to take Specialist or the equivalent, because it's dominated by people who are crazy about maths). The trickle-down effects of this might in turn explain universities slowly losing maths majors... Either that, or more maths majors are looking internationally? Australia's maths in general isn't in a great state at a tertiary level, from what I hear, so it'd make sense that with an increasing awareness for internationally-based education, more people interested in maths are looking at international universities...
And in line with what Gloamglozer said, I think part of the problem is that nobody knows what maths majors do. That, and perhaps also that mathematics isn't as strongly integrated into Commerce/Economics as it could be. Make it more obvious that people with strong mathematical skills are in high demand in many corporate places.