This scenario is "supposed" to give teh Australians a chance to be on the offensive and make the Japanese player feel some of the pain. Strategically they have stolen a march on the Japanese and the situation has given them the chance to inflict a tactical reverse on the Japanese forces. There are some hidden Japanese units and their usual fanatical morale but otherwise the Aussies should get some traction here.

On the other hand, if your main strike force gets disrupted and demoralized the second they see an enemy soldier and your secondary force blunders into the main enemy force well you end up with a mess. The Aussies couldn't hold either Kokoda or the airfield and were trundled off into the jungle. A very disappointing result.