taking my money without my permission

I have a personal loan with Nedbank for a while now and I am aware that the account is horribly in arrears , i have called numerous times to make payment arrangements as i was not able to pay the full amount for a while. A year ago i lost my job however i have still managed to make some payments which i know is not much but it's something, on Monday afternoon i received an sms saying that they will be debiting my account for over R600 when I have made a payment on the 23.06.2015,I want to know who gave them permission to want to debit my account? I know that i have not given them permission as i have been paying via internet banking for over 4 yrs now, I am not happy with the fact that they want to deduct money i don't have from my account without MY permission, I can understand if I am not paying a cent but I am making payments and I am paying what I can afford right now ,I want to know where they got permission to want to debit my account as I have not given recent consent to anyone to debit my account as I don't have money to pay the full installments. <br> <br> I am a very unhappy customer