Fearing an invasion by the Japanese, the Australians raised two Commandos companies.  They were not commandos in the sense of the specialised raiders who were trained in Scotland but more akin to the Boer commandos who gave the British Army so much trouble in Transvaal.  Composed of civilians, they fought rather credibly until forced back on the Kokoda track by superior number of Japanese.

Australian regulars then entered the fight and stopped the Japanese just short of Port Moresby.  Had they captured Port Moresby, the could have used it as a staging place for invading  Australia itself.  Gradually the Australians pushed the Japanese back, recaptured the village of Kokoda and with American help, wiped them out after difficult fighting in Buna-Gona.  Stopping the Japanese at the Owen Stanley range was the first Allied ground victory against the Japanese (Marines were still fighting at Guadacanal).  

Found this today on U-toob.  

New Guinea is an interesting and often overlooked part to the Pacific Campaign.  MacArthur, for all his many faults, conducted this campaign as well or better than any theater commander in WW2.  He did more, with less, and suffered fewer casualties, than anyone else.

They can thank Lexington and Yorktown for the Lae/Salamua raid and Coral Sea for turning back the Japanese invasion of Port Moresby

Youboob channel 'Kings and Generals' has an ongoing series about the Pacific war, this campaign having already been covered. They are excellent and detailed primers.