The minimum spacing on final in a single runway mixed mode environment depend on aircraft type, weather (vis, base, headwind) and normal occupancy time for the arrivals and departures. You use Brisbane as an example. If you have 2 turboprops arriving and are to depart a turboprop in the gap, you're going to want a 4nm gap. If they're jets you'll need 5. And if they're heavies (apart fr the mighty 767) you want 6nm. In good weather the arrival rate will be about 24 per hr. One every 150 seconds. This equates to about 6nm gaps, and allows for one heavy, or perhaps two turboprops. In this configuration you have 24 arrivals and maybe 26 or 27 departures given a reasonable traffic mix for a total movement rate of 51 or 52 per hr.