Theses are terrific, get a weekly ticket ( about $45,)which allows you travel all over Sydney on all public transport. Spend all day on the ferries. From Circle you can go everywhere at 24 knots, to wonderful destinations, take a trip all 'round the harbour...it's super value. There are pamphlets for each trip at the various stations. tickets are got from machines, get the person in the booth to help you figure out how to use them. some trips take an hour, so take coffee and sandwiches. Just be sure you know what times the ferries are coming back at if you are going to a particular destination. Cockatoo Island, Watson Bay etc etc...otherwise you'll have a long walk!!
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