My statements are not intended to critique your analysis and assumes you are correct, so just trying to provide another perspective.
Is your time horizon actually 2 years or could it be 2.5 or 3? It wouldn't be so pleasant to put your entire $10k in to options that will expire in 2 years only to see the crash happen 3 months later.
As ironchef already noted, once other investors wise up to the impending doom, the puts are going to become quite expensive. That they're not now means you're ahead of the curve. In order to get a good price on the puts, you need to open your put position on before everyone else realizes there's a massive problem. Waiting gives you the benefit of being able to extend out the time horizon for the crash, but has the risk that everyone else will realize it's going down and the puts will be more expensive. As long as you're still ahead of other investors, you'll get a good price, but since you're not finding liquidity in expirations 2 years out, waiting until it's not so far away may bring you liquidity and/or better prices (or at least a fill). Just another perspective on how to approach.
I do agree though that the bank going down will be a systemic. Do you have sector based ETFs or other indices in which you may be able to find better liquidity, but will also take a massive hit? What I mean is, if this banking stock goes down, will it cause an issue with other banking stocks? If so, a banking index or ETF might be worth looking at. How will exchange rates be affected? Will CBA failing cause another company, with possibly a more liquid options market, to also fail or be substantially impacted? For example, if this bank fails, then other businesses that rely heavily on debt will probably have a very difficult time getting capital to operate their business (even from other banks), so they may also take a large hit.
If what you're saying is true, that the housing market, exports, etc is all going to be a problem, I think it's wise to look at everything that may be impacted rather than focus on a single stock (particularly because the market isn't giving you what you want -- low strike, long dated, cheap puts), so you should look at alternative methods to capitalize on the same thesis.
Good luck.
Thanks for your reply.
I need to think about the strategy itself a bit. I have a couple of ideas that I will work through and see if they make sense.
I like puts because of their properties, especially the limited risk, and leverage.
I am aware of some other shorting mechanisms. There are inverse ETFs on the ASX which are funds holding cash and futures against the ASX200 index. There are two that I know of inverse leveraged at 0.9-1.1 and 2-2.75. Another way would be gold to short the AUD. Short selling but this is too risky for my tiny amount of capital. Holding cash and just waiting.

