Even if there isn’t a specific number, the ballpark would be great.
First of all, as you may know Ivy League institutions do not publicly state the minimum requirements for candidate GPA, let alone ATAR “cut-off” for Australian students. That being said, given my experience in working with Aussie students who apply to top-tier US colleges over the last five years, I’d say 99 would be a safe bet.
One thing you need to keep in mind is that getting into the Ivy League or other prestigious US colleges does not fully depend on your school academics, as the process is significantly more holistic than getting into the Group of Eight in Australia. As a result GPA is only one of the factors that admission officers consider and there are a range of other things that they take into account, such as your standardised test scores, extracurricular profile, leadership, essays and interview. That’s why I had some students in the past who got into Ivy League with an ATAR lower than 99. It is still possible if you have a strong application overall.
Hope it helps! Let me know if you have more questions and feel free to follow up with me.