Not informed about transfer between account

I have recently opened a cheque account with Nedbank as I had an account 2013. I saw a payment on my bank statement of R916 that was deducted 31/11/2019. The next morning, I received a sms informing me that Nedbank transferred R916 from my Cheque account to my personal loan. I made no arrangement for any money to be transferred or to be deducted. When I ask who gave them authority to take money from my account the said that they could not get hold of me and the send the last sms in 2016. I am disgusted in the way this was handled and feel like I had no say in the matter concerning my money and account. I also don’t know if it is legal or not, but I will seek advice. Marius Snyders ***