@backstreetboy might be able to tell you if there is a month-to-month version or not.
Telkom also offer a 1TB deal but for R999pm, You may have to go into a store to get it at month-to-month though.
For those with good signal to a 2300MHz tower without any congestion, I'd recommend Telkom.
This is like buying meat at the cornershop when there is big butcheries all over the town. Don't think it will last, but who am I to say.
Expect numerous calls from call centers, trying to sell this deals.