The minimum connection time for international to domestic at JNB is 90 minutes so it's surprising that any agent would allow, or even be able to book, less than that.
Personally I think 80 minutes is just too short. You must check your luggage onto the domestic leg at least 30 minutes before departure. That leaves just 50 minutes to clear immigration, collect luggage, clear customs and get to a check-in desk. It's certainly doable but it really doesn't leave much room for anything going wrong.