The only official way to drain "drain an opal" fully is to cash out to an Australian bank account.

If you don't have one, you need to work out the fares and just get it as close to 0 as possible. Otherwise you need to plan carefully: if you get the balance down to an amount slightly over the minimum fare from a station, tap in at that station, then take a trip that costs more than the minimum fare, the card would go to a negative balance when you tap out. Then you can chuck it. Needless to say, if you get inspected by an officer when leaving the end station in this way, you might get fined $200.

Also not sure if ticket barriers will open when there is a negative balance or you have to be let out manually then forced to top up (not all stations are barriered). People used to do this to exit the airport station and evade the airport access fee.