@buffy9 nails it
here
Long range air defence / anti-ballistic missile defence is the least effective way of spending government dollars. Robodebt 2.0 would be better. Replacing all the trash cans in Canberra with orange ones would be better. Hell, just stacking a pile of money in a field and setting it on fire is a better option.
It is phenomenally complex, and honestly, the missile is the easy part. When you play with the orbital mechanics and likely incoming flights, there are sheer impracticalities. For instance, depending on your threat/missile/sensor match up, your cueing sensor (to defend Darwin) has to be located in the middle of the SCS. Going out on a limb - but if it's Beijing's PLA-RF shooting at us, it's unlikely they'll let us park something there. And what are we defending? Against a conventional missile - honestly, what does it matter? A Hwasong-12 carries a 650 kg warhead - what will that destroy? A city block? It's harsh, but 650kg of HE is meaningless.
If it's nukes you are talking, that's a whole different ballgame. Nuclear theory, diplomacy and the like is far beyond this forum. Yes, a nuke hitting an Australian city would be horrific. But (a) even the PLA-RF have limited numbers of nukes, how likely is one being thrown against Australia? and (b), the international fallout would end the shooting nation. I'd argue the likelihood of a nuke being thrown our way is so low that we can continue to rely on the ambiguous coverage of the US.