Unauthorized money debited from my account

I have a loan with Nedbank and every month they deduct R7800 which is they amount that has always been deducted but for the past 4 months I have been receiving Debi check mandates from them and I always reject them because that is not something I was told about,Nedbank went on and took the extra R3000 on top of the R7800 without my consent and when I call their customer service I get transferred from one agent to another in a call that would last for close to an hour without finding any help,so in actual fact Nedbank is *****ing money from me and some of my debt orders are bouncing because of that.