Simon, I had to register for GT when my turnover exceeded $75K... but thats in my line of work and I am not sure about retail.... the following link has the regs etc but worth talking to an accountant, business and or tax adviser to be sure and to be sure you have the right reporting mechanisms in place... good luck... DL
http://www.ato.gov.au/businesses/pathway.asp?pc=001/003/022&mfp=001&mnu=60#001_003_022
