And a senior government official, attempting to flee to the redoubt by ship, discovers it has already fallen and diverts to Marion Island...
For an Apartheid official it's more likely airplane. I just thought Cape Town because while a lot more Anglo dominated then Jo Burg or Pretoria it does have sea access with a port, a population base and some geographic features useful for defense (Table Mountain would make one hell of a protected FOB for South African heavy artillery and aircraft. Planning for expulsions of "Undesirables" and lots of refugee whites (preferably for the Regime Afrikaaners.) and a economy of total crap but it might be their best bet. Inland Last Redoubts are too vulnerable to being cut off and suffering logistical nightmares. That is unless their able to secure deals with say new or existing neigbhoring black states to effectively use their ports.
Maybe add in "Redeker Zones" of fortified eternally besieged smaller redoubts inland designed to attract as much hostile attention as possible and save the main redoubt the extra attacks.