Airport Coordination Australia allocate slot times, that if everyone met, would ensure the 80 an hour cap would not be breached. But in everyday operations, some run a little late, some a little early and some small delays in one 15 minute block can give the tower a few more aircraft to depart in the next block(s).
As long as the arrival load is full, and the departure queues are full, the cap can be easily broken, but it involves full loads (and a good sequence of aircraft) for a full hour.
We regularly have "cap threat periods" where the movements are counted and may have to slightly delay a departure or two until the next 15 minute block.
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