Capitec digging fingers into client funds unconsented.

My bank does not have any overdraft facilities available but today I received an sms advising me that I owed an amount, and the bank has made a decision on my behalf(no consent from me, whatsoever), to offset the amount-that’s basically taking money from one of my accounts to pay a negative balance on another. I need to find out on what grounds this was done and how possible it is for this to be a legal transaction as it was not authorized by the account holder. If this is being done, what else can the bank do without my consent??