I don't believe any power on earth (bar the US) currently has the power projection capability to put a sufficient force into the sea air gap to seriously challenge the ADF on our own ground (and water), period. In any case such a move by a major power (PROC is the only possibility as far as i can tell) would have massive consequences for the wider security situation in the Asia Pacific region and for the US's alliance system, thus you could expect a swift and decisive reaction.
If however in the 2030+ time frame the US is unable or unwilling to defend Australia in a wider conflict with another major power then the alliance may not be the panacea it is now. (But of course that is dooms day stuff). If the regional balance of power significantly alters in favor of other major powers then the ADF may have to address a fundamental shift in our strategic environment, one never faced in the history of this nation. We have a long history of relying on "great and powerful friends", what will happen if our current friends are no longer able to defend us? Will we look to a new alliance system or to our own defense at a wider regional level?
It's interesting. Some thoughts, if its is okay......
Singapore and Malaysia have no reason to attack (territorially etc) and Indonesia seems to be the only
real potential issue, However, were someone like China to be on the cards, I think we can expect several issues to come into play:
A) China has no real reason to attack beyond conquest etc (no direct military threat to the mainland - perhaps limited to punishment strikes to disarm or inflict retribution for supporting 'interventionist western forces). Raw material possession may be a factor, but we already sell it (although I admit that I don't think this stopped the Japanese in WW2).
B) Any attact on Australia implies freedom of movement through the seas with forces capable of strike or bases sufficient to facilitate use of airpower. In relation to the former even a single nuclear submarine would be a problem without sufficient MPAs and therefore suggest that increased numbers would be required and in relation to the latter, that suggests the acquiescence of regional states and therefore a proactive policy of diplomatic denial ought to be the tactic of the day.
C) Threats from China that require transit/passage through the region depend a little on the outcomes of territorial disputes from areas such as the South China Sea. Smaller states tend to gravitate to the dominant powers of the day (in this case US, ealier British Empire and potentially in the future - PRC). As such it cannot be assumed that ASEAN nations will necessarily be persuaded to join in any regional alliances unless diplomatic relations with the PRC sour for some reason (Disputes etc). I think we can rely on continuing Japanese friendship as a regional competitor, but but regional coalitions/alliances may be restricted to peace within the South East Asian regional nations only. It may be a little to much of an 'ask' to expect that regional states in the 'zones of primary transit' would proactively attempt to restrict PRC's freedom of maritime manouvre.
D) At this point the intentions of the PRC and India become important. Three potential scenarios for CHINA might be:
ONE the Chinese government forms the impression that it will persue an introverted policy of self-focus, assisted by international diplomacy and action to secure resources and trade then conflict is unlikely unless provoked (ala Taiwan).
TWO, great power with interests vested as a stake-holder in the current international setup - proactive action within the scope of the will of the international community with only the occaisional foray into proactive military action (limited in scope and duration - full expection of Taiwan initiated reunification) which augers well for Australia. A peaceful border resolution with the Russians is an example of this.
THREE, proactive great power - where the growing capabilities of the PRC are combined with the political desire to project an image of power for international and domestic political reasons. Forced resolutions of disputes presents the greatest potential for great power conflict. Australia is drawn in with western allies against China (perhaps the only realistic China threat scenario for Australia). At this point it seems to depend alot on the timeframes involved and whether the conflicts can be confined to the NEA region and whether the use of nuclear weapons can be negated (either by deterence, premptive strike or BMD). The indian position becomes important as a result of the potential of the Indian Navy in particular in the future to develope into a force capable of preventing Chinese expansion into South West Pacific and more likely the Indian Ocean.
All in all the prospects for avoiding war are good so long as there is no reason for the US and PRC to be drawn into conflict or where disputes are negotiated and positive soft power draws in the 'rebel' Taiwanese. If the India/PRC borders become an issue that is a whole new ball game and I am not sure what would happen in this scenario.
Regards,
Brett.