Overcharged for data

I have been a MTN contract customer for probably 15+ years. My current data cost on contract is R160.00 for 1GB data. This past month I have unknowingly gone over the data cap and they have advised me that they have billed me 'out of package' rates at R0.99/MB which they say equates to approx. R990.00/GB!! I am happy to pay for any data that I have used but how can MTN justify a 600% increase in price for 'out of package' data? As far as I am concerned this is abuse. This excessive usage happened over a single day so this was not high usage over the month. I am also unable to verify how my data usage was so high. The only feedback I have got is that they have verified it was from my SIM card so pay up. Now I have a data charge for R4,992.00 which should have cost me around R800.00. As soon as my contract is up I will be moving to another telecoms provider.