Hi everyone, thanks for your replies

Below is the response I got from Australia Post, which I've passed on to the buyer. I'm going to leave it in his hands as to what he wants to do. 

Hi Neil,

I'm so sorry to see this! How disappointing for the both of you. We'll be happy to help however.

SX2HGNT isn't valid in our system, could you please double check the tracking ID for the parcel?

To allow us to investigate, please ask the addressee to take the contents and any packaging to any Australia Post Shop. Please note that our LPO locations aren't able to process these enquires. You can find your local Post Shop by visiting our Locations & Hours here, http://auspost.com.au/pol/app/locate.

The staff at our corporate Post Shop will help them complete a damage assessment online form which we'll then receive so we can assess the claim for compensation. While the claim is processed, we'll need to keep both the packaging and the contents of the parcel at the Post Shop for the duration of the investigation. Once we've received all the required information from the Post Shop, we’ll be in contact with them within 10 days with an outcome. - Kiona

Thanks, and have a merry xmas everyone, 

Neil