"could anyone tell me in which piece of legislation it is written that the seller upon the sale of goods (or service) must produce a receipt if requested to do so by the buyer ?"
I don't have on hand the actual section within the GST legislation that deals with the requirement to provide a tax invoice when a customer requests for one, but it is within Subdivision 29-C of the GST legislation that deals with the subject matter of "tax invoices".
The requirement is GST specific, has nothing to do with Trade Practices ACT 1974.