Firstly, has the buyer sent you pictures of the broken item?

As long as you sent with tracking, you get up to $100 in cover. Extra insurance would be for things over $100.

The best way to go about it is to first get clear pictures from the buyer.

Then contact Aus Post through a PM on their Facebook page. Be nice and friendly and explain the situation in brief. Give them the tracking number and upload the relevant image/s.

They are pretty busy at the moment, but usually get back to you within 24 hours.

If the buyer is reluctant to send pictures, that would be a red flag.