How IOUs work An IOU is applied when there is not enough money on your Snapper to cover your tag off fare. You can incur an IOU on your Snapper when: You don’t have enough money on your Snapper when you tag off. You don’t tag off and your Snapper is charged a penalty fare. You travel outside of the zones that your Snapper travel pass is valid for, and your Snapper is charged an extension fare. You will need to top up your Snapper after you incur an IOU so that you can use your Snapper on public transport again. If you use a travel pass it will not work until you have cleared the IOU. The IOU is removed from your Snapper (i.e. you pay it back) the next time you tag on. Like most things in life, prevention is better than the cure. To avoid incurring an IOU on your Snapper and making your travel pass invalid you should always have some money loaded on your Snapper to cover one or two journeys on your regular commute. This is also handy in case you forget to renew your travel pass and you still need to travel using your Snapper.