If Australia invests funds in BMD capability it is a solid little victory for N.Korea and China. Because those funds would otherwise be spent on the Navy or Airforce.
In fact if I were an intelligence officer for a country like that, it would be my goal to cultivate a false sense of ballistic threat (through faux test launches, fake intel leaks and false claims on propagandist media) to a laughably remote country to make my enemies squander resources and train to defend an attack that isn't included in any of my country's military plans anyway.
North Korea is on a completely different tactic (IMO), they want cash and concessions in exchange for mothballing their programs. So its important every so often they announce new capability or some new weapons system or fire a missile or test a bomb. As we have seen their fusion weapons seem highly suspect, but they appear to be embarking on that path. North Korea will sell on to anyone at any time who has the cash. They really do bring chaos to the table. But realistically they aren't a huge threat worth worrying about. They aren't globally ambitious, they are limited in resources. They are a headache for S.Korea and Japan, and China (and Russia) but really that it. Obviously North Korea launching a Nuke at Japan would be bad for everyone, its conceivably possible that they would do it, and the US, Japan, Korea, and even China would do everything they could to stop it.
When people talk about defending from N.Korean missile (in Australia) the subtext is a Chinese threat. China is way more capable, has reliable launchers, large numbers of them and is very concerned about its region and territory and areas around it.
The US isn't developing SM-3 for NK, its about China. Which is why the SCS is very sensitive, if your going to intercept a BM from china to the US you want to be located in the SCS. China wants to control the SCS.
For Australia, its about being a strong partner in the region. We do bring technologies to the table such as JORN, we have compatible ships, we have aircraft that would be useful. We are one of the few countries that are already part of the "Systems of Systems". We can't make use of SM-3 capability singularly by ourselves,we have too few ships, too far away (although that may change with the future frigate), but we can certainly play a key partnership in a multinational BMD shield.
An Australian, Japan and US alliance and BMD shield is much better than just a US BMD. For one thing its not unilateral. Costs are shared, responsibilities are shared, it fosters the alliance, more allies have more capability. You can't look at it from a purely technical standpoint you need to see the bigger picture.
That being said its not the most pressing need for Australia at the moment.Its bleeding edge technology that won't be effectively fielded until 2018 by anyone. Its the sort of thing we need to look at in the future, once we have the AWD in the water and operating (and a million other things the ADF is trying to do). 2025-2030 IMO would be when we should look hard at incorporating SM-3 as part of a comprehensive AW package. Not having SM-3 may mean in the future we can't screen for US assets in a multiforce mission (like Timor) or help secure the Asia pacific region.
I think this diplomat story sums it up pretty well, Australia, Japan, Korea and US.
Aegis, Missile Defense and the US Pivot | The Diplomat