So the bombing of Darwin was not the responsibility of the RN to protect the borders of Australia? Help me out here.
The RN suffered major losses trying to contain the IJN/IJA, and prevent them from reaching Indonesia, where they could pose a threat to Australia. However, after that the Allies divided the world into theatres of operation and it was the RAAF and USAAF that were nominally responsible for the air defence of Australia. As I stated Allied intel knew the general intentions of the IJN/IJA well in advance, and thus the RN was not ordered into Australian waters in 1942.
In 1942 RAF fighter squadrons were ordered to Darwin to help defeat the IJN/IJA aerial attacks.