As I understand it, you should get your old role back if it still exists.
Did they say why they cant offer you your job back? If they have given it to someone else, then that is totally wrong and you have the right to demand it back. They should have filled it temporarily and told that person that it was a temporary job for the time you were on leave. As far as I know, they cant offer you job on a permanent basis to someone else unless you have resigned.

Have a look on the industrial relations website. It explains it all there. Maybe even give the Industrial Relations people a call and tell them about whats happening. I made several calls to them when my work was trying to stuff me over regrading returning to work. I hope you can get something sorted out.