Updated 6/3/2015 for new 2015 fares
Due to the way Opal implements minimum required balances for travel, it’s possible for your Opal card to fall into having a negative balance.
To travel with Opal, all that’s required is the minimum balance at time of tap on.
The minimum value to tap on with Opal depends on the time of tap on and the mode of transport. For trains, the minimum balance is $2.36 during off-peak and $3.38 during peak hours. For ferries it is $5.74 and for buses or light rail, $2.10.
Of course, you will need to top up again in order to use the card, but once you’ve tapped on, you can travel anywhere and tap off.
With the announcement that now you can purchase an unregistered opal card from a retailer with as little as $10 cash, this makes this option slightly more feasible.