Without the benefit of hindsight and what happened historically in WW2, which direction is it in the 1920s and 1930s that the Japanese threat to Australia is expected to come from in this scenario?
The broad expanse of the Pacific off Australia's East coast?
Through the South China Sea or Dutch East Indies?
Where exactly do you intend that any battle between the IJN and the RAN will actually take place in this timeframe?
Is it intended to augment the RN China Station or fight its own war?
I don’t believe the RAN has ever seriously considered operating independently. Before December 1941, Great Britain was its strategic guarantor; after Pearl Harbor, the fall of Singapore, and the Royal Navy’s temporary withdrawal from the Far East, that role shifted to the United States.
In terms of immediate national defence, the RAN’s primary focus has always been securing continental Australia’s northern approaches—essentially, our own ‘First Island Chain.’ This extends from the Bay of Bengal in the west, through the Indonesian Archipelago, Papua New Guinea, New Britain, and eastward to the Solomons, Fiji, and New Caledonia.
Beyond that core task, cooperation with the Royal Navy—whether through the old 'China Station' or the 'Eastern Fleet'—was the logical course of action in countering any threats in the Southwest Pacific. However, pre-war contingency planning also factored in strategic collaboration with the US Navy, albeit largely through the agency of the RN rather than a direct RAN-USN relationship. Interwar strategic assessments reflected this, as British planners increasingly recognised that countering the Japanese would require US cooperation, particularly as the Euro-Atlantic security situation deteriorated during the 30s and the envisioned ‘Singapore Strategy’ failed to materialise as originally conceived.
By late 1941, efforts were underway to coordinate an Allied response to Japanese adventurism, though these ultimately proved ineffective. The ill-fated ADBA (American-Dutch-British-Australian) Command was one such attempt. Another was the mission of Admiral Sir Tom Phillips, C-in-C Eastern Fleet, to Manila on Dec 7/8, 1941, for discussions with Gen. Macarthur and USN leaders—immediately before returning to Singapore to lead the doomed sortie of Force Z. HMS
Prince of Wales and HMS
Repulse were sunk two days later on 10 December 1941 starting the collapse of the house of cards which was the British position in the Far-East by that point.
Convoy protection across the Indian Ocean had long been a joint RN-RAN responsibility, as seen in both World Wars. Meanwhile, escort duties along Australia's western coastline and through the Dutch East Indies to Singapore were primarily an RAN tasking south of the DEI, with RN forces taking over at Sunda Strait. A notable example of this operational pattern was the lead-up to the
Sydney-
Kormoran battle on 19 November 1941, when HMAS
Sydney handed off the troop transport SS
Zealandia to HMS
Durban at Sunda before turning south toward Fremantle—later encountering
Kormoran with mutually destructive results.
In short, considering the RAN’s role before 7 December 1941 without accounting for its place within the broader British Commonwealth defence framework is, in my view, an incomplete exercise. The RAN has never been a stand-alone force [it isn't today] but an integral part of a larger coalition presence in the Indo-Pacific, structured to act in concert with [1st] British and, increasingly [post-1941], American forces in both peacetime planning and wartime contingencies. In considering a RAN 'deterrent' capability [whether through the operation of a capital ship - like HMAS
Australia - or a force of heavy/armoured cruisers - such as the theoretical '
Tiger Cubs' or the OTL
Australia [II] &
Canberra - as had been discussed herein], this reality is an inseparable part of the discourse.
