ANU can't really be placed in the same league as any of those schools lol. Also note that Ivy League is actually a sports league comprising of 8 schools, and the belief that a school is great just because it's Ivy is a widespread misconception. Each school should be evaluated based on their specialty.
To answer your question, I'd say the numbers are extremely low. Firstly, very few even apply for study overseas. Secondly, of those that do, not many would make it in. The reason is because unlike Australian uni's which only consider ATAR, US institutions consider your SAT scores as well as extracirricular activities and how well-rounded of an individual you are. Getting a high SAT isn't very difficult, and once you get above their minimum requirement, they'll look at other things. This means that it makes no difference whether you get a perfect 1600 on the SAT, or a 1550. Once you get a certain mark, they'll look at other things.
The reason why this is important is because most students tend to drop all extracirricular activities in yr11-12 to focus on maximizing their ATAR (after all, that's the only thing you need to get into uni in Australia). So whilst students in NSB and JR might be smart, most of them are purely academic-based (I'm generalising here, of course), and that's not really sufficient UNLESS you have Olympian-level talent.
I have friends who wemt to Cambridge, MIT, Princeton, Harvard, etc, based on academics alone. However, the common denominator was that they were all math Olympians. Also, only one of them came from JR/NSB. While JR and NSB have the highest average performance amongst its students, they don't have a monopoly on olympian-level students.
So I'd say that the percetange of students from JR/NSB who get into top tier US schools is similar to that of any other school, which is very low.
