Apologies if this has already been mentioned but when you have one controller firing a/c off with traffic on a 4 mile final and yet another making you wait when traffic on 5 mile final, I don't believe Perth is operating at its capacity ( nor are some controllers ), certainly not compared to some airports in Asia, Hong Kong for example. Yes HKG has parallel runways but that does not mean a/c can take off and land simultaneously ( due to possibility of overshoots, and where they are routed ).
HKG also has a plethora of ridiculous airspace constraints put upon it by numerous Chinese airports nearby and china airspace in general, with this in mind they still handle up to 60 movements an hour ( predicted to go to nearly 70 within 3 years ). Another good one is holding, or reducing to ridiculous speeds to then be told to increase speed significantly, or arriving into PH airspace to find all you can hear is crickets, no, it is not rare.
Putting the flak jacket on here but is it at all possible that with ATC being a government job, there is no incentive to get creative or make work a bit harder for themselves by managing more movements ?