SimonBl, out of interest, how much do you know about airlines and schedules?
Let me lay it out for you. Let's say the bays are full- not an unusual prospect in Perth- when the 5nm alert occurs. The alert lasts for 40 min- not unusual either. In the mean time, five aircraft arrive for those bays that were scheduled to be vacated by then. So aircraft 1-5 are now up to 40 min late as are the arriving flights. Those arriving flights all arrive on bay at the same time 40 min late. Half an hour later the ramp goes off due T/S again for half an hour. These aircraft are now more than an hour late departing and in the meantime, another bunch of arrivals are more than an hour arriving at the gate- with the next arrivals landing as they park.
In short, when you're operating a terminal at virtually full capacity- as most Qantas terminals do- then it doesn't take many thunderstorms before there are going to be multiple hours of delays to arriving and departing aircraft.
Why doesn't this affect DJ to the same extent? They're probably not operating at terminal capacity like Qantas is at Perth. They have more bays available and a lower flight frequency thus not compounding the problem as quickly.
The longest delay I've heard recorded thus far was Newman- Perth. 1:35 flight time, 8:24 to get from touchdown to blox.