I'd say the best form of defense for Australia is to be a peaceable, useful and worthwhile neighbour (with a 'u') with a good culture of trade and fairness so our fellow nations in the region have more to lose than to gain by being aggressive.
Our expansive and inhospitable northern inland as well as the distance from everyone else is a good defence system, although of course augmenting it might be a wise thing
On the other hand, being a one-eyed supporter of debt-financed US imperialism is always going to be more expensive and a greater threat to the nation's wellbeing.