Though they've already been mentioned, I've got to put in a recommendation for Hirst Arts molds, been using them for several years & have got to say they've been the single most useful gaming purchase I've made. The ability to produce excellent looking scenery in quantity has been fantastic, as casting with plaster derivatives (like Stone Plaster or Herculite, both commonly available & cheap) is quick & easy (it's also possible to cast using resin with them, though this will degrade the molds eventually). The molds themselves are pretty reasonably priced (especially when you consider the virtually unlimited amount you can produce from them).
Your only problem, I imagine, would be the postal charges to Australia, so looking for a stockist there would be the first step, good luck with the scenery project.