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According to Avis in Mauritius:
At the time of rentals, the driver must present a valid drivers license in the drivers name from their country of residence. Customers presenting a drivers license in a non-Roman alphabet must also present an International Driving Permit (IDP) to serve as a translation in addition to their valid drivers license.
So an IDP is only needed if your driving license was issued in a non-Roman alphabet, which won't be the case for Austria.
Likewise Avis in South Africa says:
At the time of rentals, the driver must present a valid drivers license in the drivers name from their country of residence. Customers presenting a drivers license in a non-Roman alphabet must also present an International Driving Permit (IDP) to serve as a translation in addition to their valid drivers license.
So no IDP is needed for South Africa either. Compare this to Avis in Tokyo:
You will need your passport and International Driving Permit (IDP), recognized under the Geneva Convention. The IDP must have an expiry date of one year after issue. It will be valid in Japan one year after entering the country (immigration stamp).IDP recognized under the Paris Convention (1926), Washington Convention (1943), Vienna Convention (1968) are not permitted in Japan. *IDP issued in Japan will not be accepted. If you hold a driving permit from Switzerland, Germany, France, Taiwan, Belgium, Slovenia, Monaco, you may use your driving permit in Japan for one year after arrival, as long as you provide a Japanese translation of the permit. Please bring both documents and passport to our store.
As you can see, the Avis website is well up to date on the local requirements, so if Mauritius or South Africa actually required an IDP to be used by foreign drivers, they'd have made this a requirement for renting a car from them.