Like most (now privatised) airports in Australia, or roads or rail - it is lack of infrastructure investment
Your dead right here. The problem doesnt start at the 21 holding point. The problem starts 5km further down the Tonkin HWY, Leach HWY, Great Eastern HWY or any other road leading into the airport.
Then you have insufficient Check-in counters, a joke of security (who seem more concerned about getting there 50% of crew quota), and a departure lounge that is being increased in size to cater for the pax numbers of 2 years ago, and not now or future growth.
The whole airport is an embarrasing joke.