Autopage
How can you ask almost R1000 per GB? Is this legal

I have a contract with Autopage for 1GB data per month. My internet is supposed to switch off after 1GB are used. But this month, my data did not switch off and I used my internet as usual, thinking that obviously it will switch off. Not this time, it ran and I received a billof almost R6000. Stating that I used6GB. How can they charge almost R1000 per GB? Is this even legal? I cannot pay that amountof money. I mean, I have an agreement with then that my data will only be 1GB per month and after that I will top-up myself?
PLEASE HELP!!!