Earthjade said:Constitutional power s51(xii) gives the Commonwealth power over "currency, coinage, and legal tender"
So they can legally make fiat.
If they have power over "currency, coinage, and legal tender" they can legally recognise whatever they like as "money", gold and silver included.
[Edit]
Put it this way, there is nothing in the constitution to prevent the Commonwealth government passing a bill called Livestock as Legal Tender Act 2012 and letting you pay your income tax in chickens valued at AUD$7.45 each.