Thanks for the replies, it was just the rear driver side that did it. All the others were showing up correctly. They are all at 38/39psi. I brought the car brand new in March and haven't touched the wheels so its what Hyundai set it too. I hope the batteries aren't going dead already. I've done about 120km since I posted this the other day and it's been fine, no more errors. Maybe it was just a once off thing.
I was more curious if this was a normal thing for them to drop out like that. I'm just about to do a return trip to Melb in a month and it could get annoying if it happens again.