twoOsix
From a reasonable amount of experience flying around the backblocks in OZ I can give the following information but I haven't seen any regs as such, but I have seen it repeated so many times that I think its probably true.
There is some regs which prohibit refuellers adding new fuel to an open drum. I used to used drum-stock and I had an extra drum that I decanted any dregs into and then called the bulk supplier who came and filled up my now empty drums and put a sticker with batch number etc on the now full drums. He inspected the drums first for rust etc before hand.
If you want to buy fuel from, say a cattle station, as you travel past is a bit varied. Places with a licence to sell fuel won't sell you anything but a full, sealed drum. They won't take back or give credit for any fuel not used. Some make you pay for the drum as well as the fuel - I have flown off and left a few drums around the outback that I have paid for but I am sure someone is now using as a bush BBQ or a feed trough.
Places where you know the owners may be a bit more flexible, like buying back the excess you don't use or not charging you for the drum itself. But its always on the understanding that its all at everyones own risk.
Other places have given me the choice, buy a full drum or I'll give you some from an opened drum. Your choice your risk.
Everybody is (not unreasonably) covering their a....s