The guidelines state that you must have "sufficient funds to support yourself during your stay and to organise your return trip" in order to enter the country on your WHV. There is a general understanding that this means around $5000AUD in a bank account as well as a return flight ticket (or enough funds for such). From our experience this is not a strict policy, with almost no one being asked to prove it upon entry, however, we didn't take the risk anyway.