That will be a difficult route - there is next to no direct traffic from Australia to South Africa for RoRo or containers. The route will be Melbourne to Singapore - trans ship in Singapore for an East Coast African ship maybe via India or the Gulf States - maybe trans ship in the Gulf as well.
Any competent shipping company in Melbourne can help - Yellow Pages for a lot of quotes. I'd have expected a US$4,500 to US$5,500 mark - but dont hold me too that.
You may not get a quote to cover port/stevedores charges in SA as this is usually paid by you on retrieving the vehicle in local currency. You need to get some idea of this - as some shippers get really stuck into you on port charges - it can make the cheapest quote the most expensive.