Particularly if there's a chance that the Archives will do this sort of thing again, there might be some potential for creating an External Link Template similar to the ones for FindAGrave and Ancestry. Those templates ask the user to supply the code(s) that identify a specific record, and the template expands the entry to include the rest of the URL. If the URL format changes again, only the template needs to be revised to get all of the URLs fixed. Before such a template is created, it would be best to communicate with the National Archives of Australia to verify your information about their coding system. A Team Member here would have to implement the template, but others can lay the groundwork. Project Leaders of the Australia Project should be engaged regarding this situation.