Hi!
Adding to what Olivia said, you do need at least a credit average and WAM cut-offs vary according to popularity.
In response to your questions:
1. WAM cut-off for Wharton
This varies every year because cut-offs are based on popularity so if not as many students go one year, the WAM cut-off will decrease. As of Semester 2 2018, the cut-off for Wharton was a HD.
2. When can you go on exchange, and how long is the application process?
After you've completed 18 units of credit (3 subjects), you're good to go! I don't know if this is just me being an International Studies student but the application process isn't that long. It's just the course matching you'll have to do that takes the longest, so the subjects you want to take at your desired overseas institution have to be somewhat similar to the ones you would've taken at UNSW if you didn't go on exchange. I hope that makes sense!
I hope this helps! Feel free to ask more questions as I'm currently applying for exchange now :-)