Catching the train from Sydney airport may now appear a less attractive transport option for visitors, after a minimum $35 top-up was introduced for new Opal cards in a bid to stop people exploiting a loophole that was costing the state government millions.

All trips to and from the Sydney Airport stations attract a station access fee of $14.30 for adults or $12.80 for concessions, but since commuters are allowed to tap on with a balance of just $3.66 and can tap off with a negative balance, airport-goers can avoid paying the hefty surcharge by simply throwing away their card with the balance owing.

The steep fee wasn't attracting many takers on Monday afternoon. Credit:Josh Dye

A spokeswoman for Transport for NSW confirmed that the $35 minimum, which covers the cost of a return train trip to the airport, was an effort to "stop the fraud which has been going on".

At the end of last financial year, Opal cards with insufficient funds at tap-off had cost taxpayers a total of more than $7 million, she said.