The SACP were a part of the ANC and still are. The other parties such as the DP and IFP mentioned support for the proposal in 1993.
Though the IFP would have preferred their own monarch instead.
Anyways according to After Mandela Buthlezi apparently told Mandela he would be amenable to a Windsor monarchy.
Now, if he had made that public, that does change the equation as far as the IFP goes. Still doesn't change the intransigence of the NP, who was still the ANC's primary negotiating partner for the transition. If the NP said no to restoring the monarchy, then the proposal would have to be withdrawn - and the ANC needed to keep the NP on side to prevent things escalating even further than it could have been, especially with the far-right and the cries for a Volkstaat.
Why would the referendum even happen in the first place?
For reasons specific to Australia, such as the constitutional crisis in the '70s, the Australia Act in 1986 (of which becoming a republic would be the icing on the case), plus Keating (a republican) as Australian PM around the time of the South African transition. Butterfly effect application would probably dictate, if Keating doesn't screw things up, that the ALP would probably win another general election and would follow through on their promise to make Australia into a republic by 2001 (which may or may not need a referendum, but I would assume some popular consultation would be needed because Keating promised such a referendum would take place). Even without a referendum (or at least, not the specific conditions that brought John Howard to power, the 1998 Constitutional Convention, and all that), republicanism was a major issue all throughout the 1990s in Australia because it was perceived that the retention of the monarchy was a hindrance, on top of the broad waves of change that were loosening or even dropping the ties to the monarchy (the 1993 Australian citizenship oath dropped all mention of the Queen and replaced it with allegiance to Australia and its people, for example). There was also a pretty good amount of support in Australia for such a change (the main reason for the defeat of the referendum was that the chosen model was not one which all republic supporters wanted - the preferred option was direct election of the President, probably à la Ireland).
South Africa reverting back wouldn't really change the equation all that much when there were other issues specific to Australia pertaining to the monarchy that needed to be dealt with - and, because of NZ's relationship with Australia, as well as a stronger, though more pragmatic, crypto-republican sentiment going on there, if Australia becomes a republic, NZ would, too. Even the royal family was expecting Australia and New Zealand would become republics. So, South Africa would not be as much of a boon to monarchists as it would be assumed.
Nonetheless for the sake of the scenario I am asking a scenario in which the monarchy is restored with a majority of the political mandate and it's consequences internationally and domestically not how possible it was. I agree it was unlikely, but it was not impossible considering the proposal was given a lot of thought and Mandela personally thought about endorsing it along with the cabinet.
In that case, then the restoration of monarchy would not have much in terms of consequences internationally and domestically, apart from being another line item in the government's budget, seeing as for the most part there's basically not much for the monarch/GG to
do WRT being a head of state, as well as the potential for violence from the expected quarters. So the restoration would be seen as something sui generis that is just for South Africa, although a bit weird, and would not have much impact elsewhere.