The biggest problem is trying to maintain operations with such a small number of airframes. The Army found when they initially had four Chinooks that it was unable to make effective use of them. They then got permission from the Government for two more - maybe this is the RAAF's plan, get a foot in the door first.
Most airlines look at a minimum fleet size of 12-15 to be cost effective - less than that makes the cost of spares and manpower too prohibitive.