One of the problems with buying a mouthpiece in Australia is that the shops tend to stock a very limited selection. More common are Yamaha and Otto Links, etc., but the sizes you want may not be there. Another problem is that many will not allow to try them out, generally for the reasons you have mentioned.
Another reason they do not let you try them out is, I think, that mouthpieces in Australia are very expensive. From what I have seen, the appreciation of the Australian dollar has not made a dent in the prices at all. People test them, and then buy elsewhere more cheaply. My solution has been to ignore the shops altogether and to work out from posts on this forum and elsewhere what might suit me, and then order from overseas. It has worked for me, but I should stress that I have not been in the market for anything exotic.
In Melbourne you could always get in touch with elthamwoodwind.com.au. I have not bought a mouthpiece from them, but I have found them them to be helpful in other ways. Allans is a big store, but I have no idea of what they offer.