Getting off a 14 hour flight from Vancouver, I was disappointed to find my Opal card did not work at Sydney international airport station. The Opal card has a $10 minimum balance, but a train ride from the airport to city costs more than $10 (the link from the "fare calculator" page to say how much airport access costs is not working). As my card had a balance more than the $10 minimum it had not been automatically topped up to the minimum $40, but was less than the cost of the train fare and so was rejected. I suggest NSW introduce the system as at Vancouver airport, where those with pre-paid tickets do not pay the airport surcharge.
ps: Even when the card has been accepted at a railway station, I noticed that after the gate opens a red "Card Rejected" is displayed.