Billy Boy and Rainey,

Consider this:

AIA will work with the airlines to promote new routes via Cairns. They have people specifically engaged for this purpose. So say for example, the create a route Auckland, Cairns, Singapore they will be able to influence Cairns airports pricing for landing charges for planes heading to Auckland. Without the stake in Cairns this would be an uphill battle.
New routes = more passengers in both airports = higher landing charges = $.

It's all about getting more planes landing more often at each airport - routes are developed by the Airlines AND the Airports. Jetstar coming to NZ is an example of this sort of deal.

PF.