Aust Post will only respond to a claim for damage by the recipient. In this case the buyer.

They will want them to take the item and the packaging into Aust Post staff for inspection.

It's very rare they will pay out for damage as they will just say it was inadequately packed and deny the claim.

And as said above I wouldn't let 1 NEG ruin your ebay sales.

A cleverly worded response and it won't cause you any harm.

If you don't voluntarily refund ebay will simply forcibly remove the funds from your account and give you a defect.

You have little other choice really.