I went to nyc a couple of yrs back, however I was in yr 9, with my family and was only there for a bit over a week
my personal opinion from what I remember is that its fairly busy but honestly its not much different from sydney, only slightly bigger and with more people. I think you'd have an awesome time, just be smart about it ie travellers insurance, dont overstay your visa cuz it'll fuck things right up if you ever want to go back. the shopping is fantabulous, the main thing that'd piss me off would be not being allowed to drink