To be clear, I don't think there is a specific legislation regarding duty free excise. But you have to remember that 90%+ spirits are imported to Australia. Some notable exceptions, but in general most are imported. Therefore , under the ATO excise laws, the excise is applied at importation. You cannot import solely to sell duty free. That, I know. Now, for some of the exemptions, (Bundaberg Distilleries is the one I'm thinking of), again the excise is applied at the wholesale level. Again the duty free retailer would not be able to access a non-excised product.
So, the excise is already applied before it gets into the Duty free store. The WET is not an excise, so not eligble for duty free status.
As for your example of purchasing 3 litres prior to departure and returning with them. The proviso under duty free status, is that the product, will not be returning to Australia. (Read, not returning in a re-sale state) in theory. So, customs upon your return, don't care where you bought it or how much you paid, in thier eyes you are importing excess of the duty free limit.
Hope it helps a little in understanding.
P.S.- forgot to mention about the TRS. As a foriegner, you are eligble to claim the excise, but with limitations. (think its $300 min and various other things) Plus its time consuming and is most common want of a traveller, limited time. Some do, but not that many as its not really advertised on a Punt Road Billboard