I don't think Simple Green is available in Australia. Pretty much every country seems to have it's own set of brands of more or less equivalent products, be they cleaners, degreasers, brake fluids, etc. I think you may need to find a fellow Australian to source something locally.
[edit:] Aaaaaaand ninja'd. Apparently, it
can be found. Somewhat tangentially, I think P&M needs a stickied thread listing Australian hobby products and suppliers. Seems like a disproportionate amount of "what is it called in other countries and how can I get it" questions come from Dakka's Australian contingent. Not to imply that Australians are somehow deficient, and thus incapable of helping themselves (lest they sic their army of vicious kangaroos on me), of course.

Perhaps a general "Common Hobby Products and Suppliers, by Country" thread would be more appropriate - there's one for bits already, but I feel these questions crop up enough to justify a parallel thread.