i have mates in the US and basically ivy leagues are literally on another level - like a degree from a good reputable uni like usyd are globally recognised but not to the extent of american unis, especially the ivy league. and while australia recognises academic achievement to be the main factor in judging university applications, in the us academic achievement isn't the only major thing that indicates university acceptance also a bunch of stuff like extracurriculars (way more emphasis on extracurriculars like being like the star quarterback the football team) and written statements and high quality written essays and recommendation letters and stuff. also for the ivy league they also have that whole deal of accepting legacies (the kids of past alumni)
if you're an australian student hoping to study overseas in the us you can't just hope your 99+ atar can carry you but you also have to apply directly and separately to each university you're hoping to be accepted in like american kids with the proofs of extracurriculars and written statements, there's no limit like on uac (i know one of my friends in the us applied to 20 different colleges around the country and he had to pay separate application fees for each and also write send rec letters and write separate essays for each)