The onus is on the pilot to make broadcasts on the most suitable frequency to avoid a collision, CASA in their wisdom has provided some advice as to which frequency, however there is no set parameters so to allow pilots to exercise judgement and good airmanship based on the local conditions.
If you're overflying an aerodrome at 2000AGL, while you may be above the circuit traffic, you will conflict with aircraft arriving and departing the circuit area. At 5000ft, you should conflict further out from the aerodrome at which time the departing or arriving aircraft should have made their intentions known on the Area Frequency. However again, this all depends on the local conditions such as aerodrome elevation, controlled airspace boundaries etc.
The system isn't perfect, but none would be.