I wonder if the airborne holding was extended to facilitate additional departures rather than due to additional arrivals? I'm not sure how arrival sequencing is impacted by departures from the same runway in Perth, but if there is say an additional few minutes between each arriving aircraft in order to get a few departures away, then reducing the airborne traffic holding in critical situations shouldn't be too hard to achieve by 'holding' a few aircraft on the ground.…..Is that too simplistic?