Had a situation with an American visitor here with me at a rural station with the opal card over the weekend.
There is nowhere nearby to top up or buy opal cards and no top up machine at the station. Luckily I had a spare the visitor could use. We topped up online however it takes up to 2 hours before the card will tap successfully after topping up online.
We had plans so had to board the train anyway and ticket inspectors got on about 30 minutes later. They said the visitor was travelling without a valid ticket and really they should fine her but I explained we topped up and tried to confirm it on my phone but as its a rural line it travels through many black spots with 0 reception. The inspector believed the story and was very nice about it and I said if I could find them later on I would be happy to confirm it. It made the visitor feel very uncomfortable and stressed due to no fault of her own there was nowhere around to top up the card at the station!
In My Mind, if you have unmanned stations with nowhere nearby to top up open and it takes 2 hours to clear they should not be talking about fining people. Sometimes you only realise when you tap that the card is low and you top up whilst you are on the train. Tourists would be even more unfamiliar. Everywhere else in the world with a similar opal system they have top up facilities at every station.
If the transit officers were not so reasonable, I wonder if there would be a case if you can prove you topped up the card. I don't think legally they would be able to enforce a fine until this simple flaw is fixed which would obviously only affect unmanned rural stations. Also obviously having nowhere open to purchase the opal cards, even temporary ones is very hard for tourists in less busy areas.
Everything is instant these days and it seems weird they can top the machines can top up instantly but not when you purchase online. Seems it would be an easy flaw to fix.
I believe the balance does get updated at the server which has the definitive account information but due to communication problems it may not propagate to the card next tap. Still, it's uncomfortable.