If you are confident that your packaging was not at fault, and it was inappropriate handling that caused the damage - such as excessive squeezing of parcel to fit into a mail box or such then you may wish to message the buyer and suggest that they seek compensation direct from Australia Post, as you are unable to do so as the item has been delivered.

I would make sure that I was very clear to the buyer that as the sender I am unable to do this myself due to AP rules.

It is the receiver of a postal item that needs to seek compensation from Australia Post. 

The recipient needs to take the damaged item, and all of the packaging the item came in to a post office, and complete the relevant compensation forms.  

I would also encourage the buyer to be very clear in the AP compensation forms that they complete about how they think the item has been mishandled to cause the damage, just claiming it is damaged is not enough, information needs to be provided as to what appears to have caused the damage.

I would make sure that I expressed my disappointment with AP regarding this situation occurring to the buyer and that I was keen to work towards a resolution through Australia Post compensation process.