Jetstar Refund Voucher Theft

My Jetstar Refund voucher has been fraudulently used by another person through Jetstar's booking system.
I am trying to contact with other people who have experienced the same issue.
Or
If you have a Jetstar refund voucher, or Jetstar giftr voucher, please check if it's still valid and has not been used by someone else.
Or
If you purchased a Jetstar voucher via the internet like gumtree etc, please check if it's legit.