I believe they do some sort of local batching of transactions then settle them all at once. You can see this sometimes if you go shopping early in the morning and you're the first person to use a particular EFTPOS terminal for the day, the UI will show a few extra steps in the processing workflow.
I'm not sure how this would work. Surely they must have to contact the card company to process the payment as soon as it happens since they have to show the accepted/declined message before the customer walks off.
