Do you mean rejected as in the terminal rejected it, or staff did? If staff, don't tell them. I hold the card in my hand, they'll ask a question like "is it a debit or credit card?" and I'll say yes if it's a prepaid MC/Visa or "eftpos card" if it's a prepaid eftpos card, then swipe and do my thing.
That said, one time the post office staffer saw the card face after I swiped and chucked a tantrum, so I'll leave it up to your personal judgement on if it's the right thing to do or not - to my knowledge there is no specific rule against using gift cards at auspost, but staff seem to apply their own opinions. The txn had already gone through in my case so there was nothing she could do except rave at me that "gift cards are for buying... things" to which I told her I'd just bought some tax and thanks for the heads up.
I don't go to that P/O anymore. I have found a friendlier one that doesn't care what card I swipe, as long as I pay. Another potentially helpful detail is that my friendly PO is a newsagency franchisee and not an auspost run outlet, and I find that makes a significant difference in friendliness and resourcefulness.
