I have no idea what you bought or exactly why it cannot be sent but my guess would be that your seller is not arranging shipping himself, as such, but is going through a third party.

Quite a few sellers in USA do that and it is the company itself that will decide if an item can be sent.

I don't know about now, but in the past I can recall reading about a list of things that had been knocked back for international postage and it included items such as a butter knife, things you'd normally think could get through without a problem.

If the company deems it as unsuitable they may not even be prepared to put it in the postal service at all,  even to return to the seller.

All that is just a guess but the main thing is you've been refunded. You might have better luck (and lower postage costs) by sticking to Australian sellers for a while.

It is up to you as the buyer to know what you are allowed to import into the country

Without knowing what you bought and by what means, how can you possibly expect other members to know?

Was the item sent through eBay International Shipping ?

It could have been stopped at the Hub for various reasons

What is the item number ?

Without knowing what it was, nobody here could tell you.

What's the item number ?

Edit. Snap GP.

Without some indication of what the item was, no one here will be able to offer much help.