From a hundred scale (which I think is what ATAR also uses) my scaled GPA to a hundred is about 99.5, if that helps.
ANU (the same TAC as USyd) have basically the same conversion for a SAT of 1370 = 95.0, which gives me more confidence in that initial comparison. On top of this, ANU indicate that a high school diploma GPA of 3.5 = 90.00 ATAR.
It’s important to recognise that getting the maximum score possible in your marking scheme (ie. GPA 4.00, or GPA 7.00 or OP1) doesn’t automatically convert to an ATAR of 99.95. So it’s possible using that to base equations off is a false equivalence.
The only way to know for sure is to ask UAC to convert it for you (I don’t know if there is a fee involved).