The Herald taps off and is able to exit the station with a negative balance. Credit:Janie Barrett "Unregistered Opal cards with negative balances cannot be recovered unless the passenger adds value to top up the card," the 2015 Audit Office report warned, saying customers can simply "discard their negative balance cards". As long as a passenger has the minimum balance on an unregistered Opal card to tap on, they can complete the journey and tap off with a negative balance. Transport Minister Andrew Constance said: "Put simply this is fare evasion and people should always travel with enough balance to get where they need to go." The loophole is most costly on the airport line stations, which are privately owned and operated by the Airport Link Company and include a gate access fee of $13.80.

The Herald's Opal card screen shows a negative balance. The card was topped up, not discarded, following the experiment. Users could abuse the system to the tune of $15.24 on a train journey from Parramatta to Domestic Airport during peak. The trip costs $18.62, but a minimum of only $3.38 is required to tap on. Despite owing money on the card, users can still tap off at the airport station, exit freely through the gates and simply discard their Opal card. A passenger could even board a train in Lithgow and take the three-hour, 10-minute trip to Domestic Airport off-peak with just $2.36 remaining on their Opal card, a fraction of the peak cost of $19.61.

Iain, a 29-year-old IT professional from Sydney's inner west, said a friend told him about the loophole. He said he had discarded "two or three" cards with a negative balance. "There's no fee to buy a new Opal card," Iain said. "Why not just get a new one whenever I need it?" He said he sees no problem with taking advantage of the system. "I'm not going to afford buying a house by doing this," Iain said. "I think it's similar to bringing my own snacks to go see a movie. It's such a small amount I figure the government's not going to miss that." Iain said he had a small number of friends who also exploited the loophole.

"With word of mouth and social media these days, it's one of those little tricks people can share around and use when it's convenient." But he was surprised to hear how much is owing in total. "I wasn't expecting it to be in the millions," Iain said. Another passenger, who asked not to be named, described the loophole as "a pretty big flaw in the system". The passenger said they had taken advantage of the system twice and said the gates "always open". "It will just go into negative. You can then chuck this card [out] and buy another for no extra cost."

The Herald tested the loophole last Friday, freely exiting at Domestic Airport station with a negative balance of $6.06. (Instead of discarding the Opal card, a top-up was later completed.) In 2014, then transport minister Gladys Berejiklian encouraged people to "beat the system" by tapping on and off to complete eight journeys between two short stops early in the week, and receive free travel for the rest of the week. "I want people to find the savings because they are there to be had," Ms Berejiklian said. But in March last year, new Transport Minister Andrew Constance announced a crackdown on so-called "Opal runners" by increasing the number of transfers within a journey from three to seven. The Melbourne equivalent, the Myki card, costs $6 to purchase. It can also fall into negative balance at the end of a journey.

Loading "Transport for NSW has been instructed to find a resolution that makes it fair for those customers doing the right thing, and by taxpayers," Mr Constance said. Do you know more? Contact josh.dye@fairfaxmedia.com.au