I got an ATAR (UAI at the time) in the mid 50's. The HSC system just wasn't a great way of gauging my abilities and it's probably the same for a lot of people.
I'll admit that it's something I've been concerned about when producing documents for interviews but it was never questioned and hasn't stopped my career with a major carrier so I wouldn't worry about it mate.
As long as you perform well in the interview etc I don't think they pay a lot of attention to the mark. It's more of a box ticking exercise to make sure you have the required subjects.