FNB debited my account without authorisation

We moved our home loan over to another bank as a first step as we've not been happy with the service at FNB for a while now. As part of the process a settlement amount was pulled. The new bond was registered at the end of July. We were told no payment would come off for August and if anything did come off, it would be refunded, as the bond was settled.

Did that happen, no! Firstly a random amount was deducted off the usual account the home loan was paid from. Of course we didn't put the required sum in there, as the bond was settled before the next payment was due. This charged bounced. Then two days later, this amount was just taken off my personal account. I did NOT authorise this and nor was I informed of this. I am now short almost R20000 for the month. I want this money refunded immediately. I can't wait to move the rest of my banking away from FNB, as this is exactly why we've been unhappy. Since when is it ok to deduct a random amount from someone's account WITHOUT authorisation?! When I questioned this on the app, firstly I get passed from consultant to consultant...not a great experience only to be told this is apparently interest and fees. What is the point of getting a settlement figure and settling that amount?!?! I want this money refunded to me.