What would happen if CBA goes bust?

[Smurf1976]

There are dodgy country where economy collapse but Australia, US and most developed country you can be 100% sure they will pay bonds as long as there are people paying tax. They just tax you more to make the bond payment

If the major banks all failed then they would take most of the economy with them. You can't, in practice, get tax revenue from a broke population with no jobs and worthless assets as would arise under this situation. Bonds are not totally risk free.

Also don't forget that the biggest risk with cash is inflation. You may have the notes kept safely but in due course they lose most or all of their purchasing power.