Hey mate, I don't think it is usually found in Oz, but here is a website that seems to have plenty of rough Golden Yellow Imperial Topaz for sale. I have never used this site so can not offer any feedback for it.
Hey, I have used this site a lot it has been very reliable for me and if you do come across an issue the support team are fairly quick to respond!
gemrockauctions.com/auctions/topaz-gemstones
On the left just select topaz rough.
Good luck let me know how you go!