Perhaps it's not a matter of who owns the contents of a parcel, but who the contract with Australia Post is between - a missing parcel investigation is perhaps based not on "you lost my item", but "you didn't fulfil my contract", since if the parcel is found during an investigation, it doesn't get returned to sender; the contract is fulfilled (and the fact that if a parcel is uninsured, often it is only the postage that is offered as reimbursement, i.e. the contract price). The seller may be the agent of the buyer, but seller is the one who contracts AP. If the contents of a parcel are damaged, then that is specifically about the contents and therefore possession / ownership has significantly more bearing.

Not all parcels contain goods purchased by the recipient, either (and I'm not entering a discussion about when ownership of gifts in transit passes some items are even sent with the specific understanding that they will be ultimately returned to the sender) - what would be the state of AP if they did not have uniform policies in these matters? (and yes, I'm aware that often their policies are not applied in a uniform way).