If an IFR clearance is not immediately available, then there is a conflicting IFR aircraft.
Not necessarily, ARFOR. An american colleague (whom I am fact-checking with) advises that flow control and sector capacity management are important reasons for requiring departure approval. Capacity management extends to the entire route-of-flight, and not just arrival (slot) control. They have more people just managing sector capacity than oz has en-route controllers.
It's a different world (the US world of aviation). A different set of circumstances. Thinking that you can just transplant it into the oz environment is, well, naive.