I think that if there was an easy/cheap/quick way of doing more air defence, the ADF would already be doing it.
I think people underestimate the current demand for air defence EO. Between Ukraine and Israel using it daily, the naval operations in the Red Sea pumping them out, and the rearmament efforts for INDOPACOMD, Japan, Korea and Europe the current demand is huge. I'm not sure than Australia with a small purchase can make the order books. On top of that, with what money? The IIP by definition is full, so you need to take something out. And it's all important.
That is a headache before you ask - what would we defend? How many batteries do we need to do such a task? There are a bunch of key locations spread of ~7 million square kilometres and any threat that can reach Australia has a huge throw weight. Israel is 21000 sq km (twice Sydney) and uses at least 10 short range and 2 medium range sites. Don't forget that not all the ADF missiles are going to hit, there are going to be leakers. Or you need to have many, many more launchers. And if you only buy, say, 2 Bty - what does it say to the threat's intelligence unit when you deploy those two? Perhaps these two are what the ADF sees as the highest vulnerability? Throw a few more missiles at each and overwhelm that Bty and just imagine what you could do!
Note that an SM-2 according to wiki is just shy of $4m AUD per missile; a Tp with one reload is going to cost just shy of $50m in bang. Assign a Bty per site, with at least one eNASAMS and you have nearly $200m in bang alone. And that's good for one fight. And that's the cheapest you'll get, while ignoring workforce and all the other bits that make up a SAM capability (remember, a Bty of eNASAMS under LAND 19-7b cost ~$2.5b.... the cost of these systems are astronomical). You might say that system x or y is cheaper - in which case you'll need a lot more money because you can't utilise the pre-existing Navy sustainment and training chain. You'll also decrease operational flexibility.
Finally, this is only the easy 'stuff'. We haven't got point defence yet (a single C-RAM unit is about $15 - 18m and covers a really narrow arc - you'd need over a dozen to properly defence a wide open airbase) or ballistic missile defence (the price of ban increases by an order of magnitude!). And what C2 system wraps all of this together?
Again, I think if this was a quick, easy or cheap problem, it would have already been licked.
I, would agree with you regarding the extraordinary expense and consumption rate of rare missile stocks for an effective missile defence. Yet, I think we still need to find a solution.
NASAMS is great, I believe we purchased the right short-medium system. But with AMRAAM and AIM9X it has no ballistic capability. Even with AMRAAM-ER (which I think is a good upgrade) it is limited to short ranged stuff. If this system is used to defend a littoral force, or a major defence facility, then it is vulnerable.
A Type 55 for instance can launch YJ21s from a range of 1,500km, or if China really wanted to, it could launch nuclear missiles from one of its SSBNs from as far away as 7,000-10,000km away.
In a hot conflict (which all forecasts now say is possible), then we have to consider the use of these types of weapons. And we have no current BMD defence outside the Hobarts. And we will be a priority target (FBW and Darwin will be on the same list as Guam, Diego Garcia, Hawaii and Okinawa and Yokosuka).
There are only a couple of choices for the big stuff, THAAD, SM3/6, and AAROW. Even PATRIOT and Skyceptor have limitations in this environment, however both are a big step up from the current AMRAAM based systems and provide effective medium ranged protection.
And we can't defend everything. We probably can't even defend everything that's important. We can perhaps defend some very core assets, that must stay in the fight after first strike.
My personal view is that if we want to have a littoral force capability then it needs to have something like Skyceptor or PATRIOT to accompany it with NASAMS. If we want nuclear submarine and major marine/airforce bases on our home soil, then we need something like THAAD, SM3/6 or AAROW. All are super expensive and super rare.
Skyceptor, in my view has a pathway, because it is cheap and is likely to become the US replacement for the PAC3 MSE. Once Raytheon establishes production (which it is in discussions with the US Government on), it becomes separate from the Israeli supply, resolving all the issues previously raised. It's very easily integratable into an existing NASAMS fire control system and radar package as it uses a Raytheon comms system. It would just need some additional launcher units (too big for the current cells at 5m long). I would suggest that we either place long term missile procurement orders with an American future production line, or consider developing in country production for AMRAAM and Skyceptor. These are perhaps the right missiles to partner with Raytheon on for local factories.
SM6 has the best pathway for Australia for longer ranged BMD, because it is now part of our inventory. Expanding the footprint of this weapon is logistically simpler than the other choices. It's still rare, but I note Japan is looking to establish an SM6 production facility that we could join with. SM3 is a future robusting that I would hope we consider.
So, yes they are expensive, in limited numbers and in high demand, but we still need to figure out a solution.