According to the information on the Australia Post website it says 2-4 weeks, has it actually been longer than that?
You also need to take into account that the box probably sat around for 2 weeks over Christmas and it may well be sitting in a customs hall in the US waiting clearance.
I sent a box from the UK to Las Vegas on 22nd December with Royal Mail ParcelForce (airmail) that should have arrived sometime between Christmas & New Year, but didn't even arrive in the States until Jan 8th and then sat in some customs hall in Texas until it was delivered on the 20th Jan.
If the guy who sent the parcel to you has a Certificate of Posting (or similar) he can email you a copy, at least you know that he actually sent it. If Australia Post are similar to Royal Mail then they won't even consider the item lost until 6 weeks have passed.
If it's been sent by surface by mistake it could take 3 months.
I wouldn't give up hope yet.
You also have 45 days to make a claim via PayPal, you don't need to be too hasty with that either.
Several people in this thread have made easy generalisations about 'foreigners' in this thread and it looks like you are being panicked into taking action that may not be necessary. To the rest of the world you Americans are the 'foreigners' and if Hollywood is to be believed all Americans are members of gangs, the Mafia or live in a wasteland surrounded by mutants and zombies and every one of you carries a gun including infants.
Why not email the guy saying you'll be filing a claim with PayPal if he doesn't send you a scan of the Certificate of Posting?