It depends how you paid. If by paypal, you have 45 days from the date of the original payment, so if that time has passed, your only option is if you funded the payment with a credit card. See your bank.
If it is still within the 45 days, and you sent the item back via a paypal approved method such as registered or click and send, open a dispute now.
If you paid by other means, you have no buyer protection, so all you can do is email the seller and demand a replacement or a refund by x date, or you will be taking their details to the police, assuming the seller is within Australia.
The last option is if the seller is in Australia and are a registered business, you have the option of fair trading in your state.
If the seller is OS you have no other options but feedback if you are still within 60 days.
It is always far better when returning an item, to open a dispute first, so everything is on record, you get the correct address to return to, and can add the details to the dispute.
