That loophole was closed in early 2019. It was a chronic problem at the Airport stations which have a unique ‘access fee’ of about $17. People would tap on somewhere in the train network with sufficient balance to get off somewhere else, but not at the airport stations, where they would go massively negative then toss the card.
Now, if the balance would go negative attempting to tap off at one of the Airport stations, people are forced to top-up at the machines to a sufficient balance to allow them to exit. Those stations are staffed at all times, so people can’t jump the barriers either for example. Opal cards purchased at the Airport stations also must have a minimum balance of $35 rather than the usual $10.
For all other trips, the balance at tap-on must be positive and sufficient to cover the minimum fare on that transport mode.