As Sammi pointed out, it does need to have a reason. I wasn't thrilled that I had to pay the entire $160+ for my 32wk scan when my GP had requested it and referred me for it.

Apparently however, there is a certain code on the referral that you can get your GP to put that will get Medicare to put it through for a rebate. Not sure what it was but the Medicare lady I saw mentioned it when she apologised for not being able to give me any reinbursement.