There is at least one PC app that lets you easily create and manage Kindle collections from the PC. Check the Kindle forum.
Pocketbook readers offer *dual* file management systems; you can use either or both. (Check *their* Forum for tips.)
The first just presents all folders in the file system that contain book files nested as deep as you want to so you can create a library anyway you want. It uses actual book files so if you want the book to show up in more than one location you need to copy the file.
The second uses a "Favorites" listing: a set of nested folders that you name arbitrarily and contain links to files or folders. So if you link to a 'New books" folder, that link will present a list of all the books in that folder. And since it uses links, and they can be copied to different locations in the "Favorites" structure, you can have your books listed in as many locations as you want. The downside is that you have to create the links manually on the reader. Not a big issue if you link to folders for categories but if you want individual book links...
Oh, and most US Pocketbook vendors *will* ship to Australia. Several PB forum members got theirs that way.