Unauthorised Debit Order

I called the contact centre after logging onto my internet banking to check my balances, I've noticed there was a debit on my account for something I didn't Authorise for R649.99. I didnt even receive an SMS. I phoned the contact centre to request a reversal, the lady tells me she cannot assist me. I mean, whats so hard to understand. The worst thing is that tomorrow morning my debits go out and now i have insufficient fund due to this, who will be accountable for this??? I'm so sick and tired of this ****. This is the reason I moved to Nedbank because previous banks was just as ***********, seriously will consider moving my account to a competent bank!