overpayment

on the 15 January 2016 Nedbank debited R5200 from my account and again R740 for loan repayment.my repayment is 740 so I do not understand why they debited 5200 without my consent.when I went to promenade branch I was told to use the bank telephone to enquire about debit .the consultant on the phone sent me back to the branch consultant so that he can call legal department for but I was told I was suppose to get telephone number and referrence number so i left the bank without any help.i just want the extra 5200 and they must continue with their normal debot as i know im indebted to them but why do they debit extra money without informing me.