Just to share my opal experience with others. I tried to top up my opal card in town hall station, and I was told I can't top up there.
Then I tried to top up online and then realised that I can't use my credit until one hour after top up. Such a strange system. It doesn't even bother to tell me my credit won't be available immediately when I top up. Found out from their staffs I have to wait for an hour.
So, make sure you have sufficient credit on your card before travel. As last minute top up will not help you at all.
You should have topped up at Woolies Town Hall. And you get 5% with the ING rebate.
If you are going to topup online you might as well sign up for auto topup.