Increase of Monhtly Home Loan Payment without consultation

Standard Bank increased my monthly instalment (Ref Query ***) SB increased the monthly payment with over R1600. I contacted them and advisefd that the installment was agreed on 1 Dec 2020. The 25 points increase in the repo rate does not add up to the R1600 increase. The explanation I received is that the increase is correct and there is no mistake. Surely it is not legal to change transactions without discussing changes and giving reasons. I request the detail reconciliation of this transaction dispute as per NCA.