Similar to Oyster cards in London and Hop cards in Auckland, Opal cards are smart cards used to travel around Sydney in public transport. It's cheaper than paying for single trip fares, and after eight trips in a week, the rest of the week is free - as long as you have a positive credit left on your card. The credit in your card does not expire, so even if you are a tourist in Sydney, you can use it on your next visit to the city. Opal cards cannot be topped up on the bus, but there are plenty of convenience shops where you can.