Off peak rail fares are cheap in NSW anyway, i.e. Engadine (Sutherland Shire) to City $3.70, so return off peak $7.40. I got an Opal Card and my travel to and from the ship from Engadine where I left my car was $2.50 with Opal Card, so $5.00 return on different days. Not a much difference from adult off peak fares, like a saving of $2.40 using Opal. But the advantage with Opal is you can go on trains, buses, ferries etc for the daily $2.50.
I probably won't use my Opal Card again unless I have another trip to the City for the OPT or to Airport, but then the Airport line charges extra, not $2.50.