A Gas n Go alternate
What does this mean?
If you're a passenger on an airline that has subscribed to Cunderdin as its alternate to Perth, then it means you stand a very good chance of actually getting to Perth within a couple of hours of your scheduled arrival time.
A gas-n-go alternate is the term that airlines use to describe the nature of their stopover if diverted to their alternate.
In our case, it means the aircraft would land at Cunderdin, and take on enough fuel to continue the flight to Perth. When the issue at Perth is resolved, the aircraft departs Cunderdin, and flies to Perth.
This is a major benefit to the airline, and the passengers. And not just the passengers on the diverted flight, but also the passengers at Perth, waiting to get on the aircraft for its next scheduled departure. And the passengers after THAT flight has landed, and so on.
The knock-on effects of a diversion are incredibly inconvenient for passengers, as well as airlines. It puts passengers in the wrong spot - parents with children are tired, and hungry, and eager to get home, or to their hotel. Business travellers on day-trips just for meetings have lost the entire point of the trip. And for the airline, it displaces the aircraft, and aircrew.
Displaced aircraft and aircrew is an extraordinary cost to international airlines, as they very often do not have duplicate aircraft and aircrew on standby. International aviation regulations limit the maximum time that aircrew can be on duty, and also stipulate the minimum required rest period that aircrew must receive before returning to work.
Cunderdin - 15 minutes from Perth Airport
If your flight is diverted to Adelaide, it could be up to 12 hours after you touch down in Adelaide, before you depart again. And that's after the ~2 hour diversion flight to get there, plus the ~2 hour flight back to Perth.
Cunderdin is a ~15 minute diversion flight from Perth. The fuel supply takes only a few minutes, because Captain is only lifting enough fuel for the ~15 minute return flight to Perth. Weather at Perth Airport is usually the cause of diversions, and it generally doesn't last for extensive periods. Once the weather cell has passed, you're on your way to Perth. Granted, not at the scheduled arrival time, but certainly much faster than a diversion flight to Adelaide (or further!).