Ah ok, if they can do that, that would probably work well. The sub thread could be renamed to say, "Australian discussion", and put a step up as a child board of Learning, Sharing, and Teaching? To make sure that new Australian's find it easily, and don't think that they can't start a non-tax related discussion?
The only downside of moving threads is if a thread really did also belong under that category. An example could be the Australian Investing Thread (which in itself is mega and would benefit greatly now in people being able to start their own threads under and Aus category).