Unauthorised Debit

My Nedbank current account is being debited for an amount each month. The source and nature of the debit is unknown to me. I have absolutely no recollection of any debit order authorised for this amount or directed to the reference mentioned in the debit.<br> Despite making Nedbank aware of this, they want to charge me R45 to place the stop payment on the unauthorised debit but importantly cannot verify or help me trace the source of the debit. I am getting no assistance or help to remedy this situation.