Theres plenty of scenarios you are not taking into consideration. You want support from our allies? well you have to give some back too, and with this upgrade we can give more than just a token hand.
Also i dont know what you are using to judge a nations defence forces as effective, but i think we have one of the most effective systems in the world. Do you really think a nation could plan an attack on us without us knowing about it? and once that happens their president, their whole cabinet and key military leaders end up dead, major highways are bombed, their electric grid is ruined, all this because we have the most effective special forces in the world and this is all before they can even get any of their forces close to ours.
On the way our high tech subs hole their ships at leasure, sneaking in, sinking what they will and out again, our 100 high tech fighters take similar action, i am not sure what exactly we are getting but i am sure their will be some jsf's and maybe even some f-22's. Both of which are the most deadly and effective fighters ever built, they dont just win 1 vs 1, they are still in a dominant position 1 vs 3 and thats if the attacking force detects them and launches enough fighters in time which they wont because they also have the most high tech stealth capabilities.
With all of this going on what kind of force would actually reach australia? in what condition would the attacking nations country be in? dont forget with all this sabotage going on everyone else will be placing embargoes and political sanctions on them, it would be suicide to try and attack us. Also nukes. Assuming we can get some, we are one of two countries on earth that can actually use large warheads in a defence manner. Oh you took a small part of the northern coast of australia? meh nothing up there anyway BOOM.
Not to mention that the whole purpose of this is as a holding action until our allies get here and clean up whats left. No, our defence plans are very effective and that is a large reason why we have never been invaded.