Have you ever tried to catch a train from one of Sydney Airport’s train stations, and encountered a ‘balance too low’ error despite having auto top up enabled on your card?

The Airport stations are one of the ‘quirks’ when it comes to Opal.

Auto top up is triggered when balance drops below $10; the default fare is taken when you tap on and amended at tap off.

For example, if your current balance is $14 on the card, and you tapped on at Central, it’d drop your balance by $8.40, bringing it to $5.60 and triggering an auto top up with the balance being under $10.

Of course, you need the minimum balance to tap on in the first place, which is minimum fare (+airport fee at airport), or $13.40+3.38=$16.78.

If you arrive at the airport and attempt to catch the train home with a balance of $14, literally tapping on anywhere else on the network would trigger auto top up, but because you don’t have the minimum to tap on at the airport, it won’t let you through and so can’t trigger the automatic top up.

Luckily, there’s plenty of Opal top up machines at the airport and you’ll be on your way in no time.