Thermionic, following the info from 123 above I found NAT3014. http://www.ato.gov.au/content/downloads/bus20724nat3014072009.pdf
"If you make taxable sales, your GST-registered customers and clients will need tax invoices from you to claim their GST credits for purchases of more than $82.50 (including GST). If a customer or client asks you for a tax invoice, you must provide one within 28 days of the day they ask you for it. You must be registered for GST to issue a tax invoice."
The second half seems to imply that all purchases no matter which cost require a tax invoice. I am no expert in tax or GST but I would have thought that the TPA1974 woulnd't have any info on GST?
Looking at the requirements for a tax invoice (I've only ever asked for ABN/Date/Price) it's amazing how many businesses don't follow those guidelines.
