The risk you are referring was real when GFC hit. Hence RBA guaranteed the first $200K in every account. Whether they can pay out all guarantees is another debate.
If the bank say CBA goes belly up, who is the "they" you are referring that get paid first? Common share holders? These are the last on the list of entitlement. Debenture holders -> Preferred holders -> common stock holders in that order. I think before any liquidation takes place, any higher up the food chain creditors will take the lion share before it slowly filters down the chain.
So if CBA gets screwed, the other big 3 are probably already gone. Your tier 2 banks would have vaporised much earlier. The reverberations will flow through to related institutions who faces cash flow immediately and take down their depended line of creditors etc.
Bail in in Oz is very unlikely and usually being pushed by folks who thinks gold=money. If you go around thinking this way, you would have missed the equity run for multi decades. Cut out the useless noises and be objected. Not directed at you but at the gurus ramming up these noises.