If you're charged duty and GST on an item you have bought online and then return it, there are only limited times when that duty and GST can be refunded to you.
You can apply for a refund of duty and GST when:
duty is paid in error
a concession is later approved for the goods
the goods are of faulty manufacture
the goods were in a damaged or deteriorated condition prior to leaving Customs' control
the goods were destroyed, pillaged or lost prior to leaving Customs' control.
Contact your local Customs office to apply for a refund, and make sure you have evidence to support your application.
If you buy something from overseas and pay duty or GST, then change your mind and send it back, you can't claim a refund on the duty or GST you were charged.