Like the above guys said, ATAR is not a measure of intelligence. It's only intended to measure the capability of how well a student is able to pickup concepts and how well they can perform under exam conditions, at least that's how I see it.
I mean, someone could be a huge I.T nerd, but only score an ATAR of 65. Not only will people think he's not a genius because he didn't clock 99.95 ATAR, it also wouldn't get him into a bachelor of computer and network engineering (unless they take alternative pathways), although he may be able to score a GPA of 4.0+.
Erm, as for the question, my dad said he was in the top 2% for specialist (they referred to it as something else back then) for the state, however, when he got to Uni maths he was struggling hard, along with many others.