By introducing this rule, the tax office is acknowledging that a loss of a private key will result in an absolute, unrecoverable financial loss.
It's at least a step in the right direction if you can provide the evidence that will satisfy the tax office. If I for example claim that I no longer own the private key of a certain address, the tax department doesn't really care.
I am obliged to pay tax over whatever the amount is inside the address. In other words, you are better off not bringing it up, which is just retarded, especially when you have to pay taxes over that amount every single year. This needs to change.
If this Australian tax rule stimulates other governments to look into their tax policies and change where necessary to cover this specific subject, that would be a more than welcome development.