Do you allow anyone else to use your other eBay account?

Or could anyone else in the household have bought something whilst you were still logged in?

Was indeed an eBay purchase?

Have you phoned Australia Post/the delivery company?

I have only ever had one once, not from eBay but a major online store, for a purchase that had been delivered two weeks prior. I phoned the store and they said they had an internal error which resulted in out for delivery notifications being sent out for past orders 

If it is not a legit email address, block it