At a guess, I would say that the seller really does not want to post to Australia so has set the postage to a level that would discourage buyers.

You can hardly blame him......delays are horrendous at present.

Although it does not appear to apply here, another reason for very high postage rates is when a seller has run out of an item and is awaiting more stock.   By making postage prohibitive they can keep a listing active and not lose their place in searches.