As a CFA volunteer and pilot i beleive, large air tankers are not suited to much of Australia from both an aviation and fire point. As pointed out above runways which can handle large tankers are few and far between. From the fire perspective retardant bombing in Australia is not as effective as they are in the US. Australia bush has a very thick canopy which 'collects' most of the drop and as a result the fire just moves along under the drop in the ground fuels, American pine forests are not as dense. Australian fires also have a tendency to spot much further than US fires meaning breaches of retardant lines are much more common.