Hi Loose Rivets,
50 GB is about 15 DVDs worth, which is quite a bit. Post is the way to go. I get heaps of things posted to me in OZ, including all manner of SD and USB sticks.
SD sounds perfect (actually several SD cards will be needed). Stick ‘em in an envelope and post them - no drama. Simple. There is no paranoia in Australian Customs and they won’t be stopped. Customs are after drugs and guns, and you can’t fit many of those in SD cards.