My knowledge of Australian schools is very limited. I have read a few things about them and that's what I'm writing here. It may be wrong, so please if anyone has some greater knowledge, do share it. My thoughts on both Australian MBAs and top 15 - 25 MBAs in the US:
1) The MBA degree is not so common and/or valued in Australia as it is in the US.
2) If you want to work outside of Australia the value of such an MBA will be quite limited.
3) If you want to work in the US as an international, top 15 - 25 schools will provide some decent chances.
Top 7 = very good changes (I reserve "excellent chances" for US citizens attending such schools).
Top 15 = good chances.
Top 15 - 25 = decent chances.
Top 50+ = marginal (negligible) chances.
Top 25 - 50? it depends a lot on the program and your previous knowledge, etc. Plus it depends on which rankings you are looking at. I, personally, use Hjort's cluster system:
Then you can check with schools are included into each cluster by checking the respective cluster forum.
4) Do check if the schools you intend to apply to in the US provide loans without US cosigners. If they don't, you may get in but will have to come up with the money by yourself.
Hope it helps. L.