The ocean is still a very big place and ships are, relatively, very small objects. Even SSNs have maximum transit speeds and all weapons have maximum ranges; even those with a couple of hundred kilometres range don't cover a great deal of an ocean. And, or course, they are in limited numbers and require real time targetting. Focal points become issues but in general there are alternative routings, which admittedly will take much longer. So while the problem is a difficult one it is not an impossible one - and there would be massive damage to the world economy, not just Australia's if there was a major war; but in that situation, which is probably the only one where anybody would seriously be able to threaten Australia's communications, the significant combatants would probably be directing their major efforts elsewhere. On convoys, they still require the enemy to come to you at a point where you can concentrate your strength. The alternative is, in most situations, probably worse.