I have just come across a lurking problem with the pre-pay system at MEL: it still relies on the magnetic strip on your card, not the chip as pretty well everyone else uses. So if the strip on your card is not in good condition, the card will not register in the readers at the car park entrances. I got hit with this recently; I wound up having to take a ticket at the entry, and sort it out at the customer service window later when I left.