The state government has vowed to crack down on an Opal card loophole that cuts the cost of travel to the airport train stations by allowing savvy passengers to tap off with a negative balance.

Figures released on Wednesday show the cumulative amount of revenue lost has hit $7.8 million since Opal was introduced, with the total almost doubling in the past year alone.

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. Credit:Janie Barrett

Since 2015, the NSW Audit Office has been reporting the total amount owed on unregistered Opal cards with a negative balance.

In 2017-18, the number of Opal cards with negative balances spiked from more than 776,000 to 1.1 million, with $3.8 million in lost revenue, for a total of $7.8 million since Opal was introduced.