The details are very vague, i doubt the touch n go is "hacked". And the news could be very misrepresented like all the previous so called "hacking" news.

This is are my thoughts :-

1. The cards like every card holds line of number maybe serial no, verification no etc.
2. Instead of the so called adding top up or credit, the serial no is constantly changed as the touch n go servers will base the value of the card by the serial no.
3. As the so called miracle machine that adds credit to the card, it just a reader and writer which is available everywhere. The serial numbers are changed constantly.
4. Value of cards can be check everywhere, over the counter, banks , shopping malls.

But of course you can have it your way with the card by hacking t n g servers if you know what o/s theyre operating on, services and port running, exploit a hole, download the application which runs the t n g, reverse engineer find a stack buffer overflow or buy a zero day exploit from some russian mafia for millions which is way unlikely.