I had checked with the local SA authorities on the matter a few years ago, my personal understanding of the discussion was that the top tailgate is considered part of the vehicle and can be open while driving.

obviously you pose the unsecured load thing and it would be dependant on a persons point of view, but really anything unsecured in the back of a van is no different to a spade chucked in the back of a ute with no tarp.

just note that this is my understanding and not the 100% black and white letter of the law - it certainly pays to check, ring the local police station and ask the question - just get the officers ID number in case you need to back up your information.