I presume you will be going through the Sundra Strait into the Indian Ocean. We have recently compiled a set of cruising notes covering the south Indian Ocean from information obtained over the years from many cruisers and the few publications that exist.
I quote you from the section we have, Australia to South Africa, via Mauritius and Reunion, as this would be the quickest route for you to South Africa.
"The recommended months to undertake this passage are April to June, to be safely at ones desired destination by the start of the South Indian Ocean cyclone season, mid to end November. Expect anything from calms to gales, mostly off the stern, on the initial leg, from Australia to Christmas Island (your first stop would actually be Cocos Keeling)and Cocos (Keeling).
From here onwards, until south of Madagascar, everyting is in your favour, a slight current, and pretty consistent trades from the east to south east, sometimes a bit on the brisk side! This strong wind makes the sea lumpy and uncomfortable. Three lovely and all very different islands can be visited en route: Rodrigues, friendly to the extreme; bustling Mauritius, and the mountainous, very French island of Reunion. It is a very debatable point on how far south of madagascar to sail?
I have done this trip twice, once very close, within sight at times, but in an almost windless situation. The snag was a slight counter current and a few fishing vessels. I wouldn't recommend being this close in a fresh wind! The second time I sailed about 150 miles south, encountering a heavy swell from the south, and a few factory ships and their attendant vessels over the shallow banks.
I would feel that around 60 - 100 miles would be about right, and from that point make a course to north of your destination, ideally either Richards Bay, or Durban, to counther the effect of the strong-flowing Agulhas current. This strategy also avoids the probability of a strong northerly wind sweeping you south. Watch the barometer, and if it starts falling it means a southwesterly is on its way. If it falls rapidly after a notherly, a strong southwesterly is on the cards, with an increase in wave heights probable. Listen to the weather on the net out of South Africa, Peri-Peri or Alistar on SSB, if you have one. Otherwise, trust your 'gut' feel. In an extreme case, very seldom though, a thought would be to run with the wind during its initial strength, seldom lasting more than 10 hours'.
The trip from Durban to Cape Town is reasonably straight forward, very weather dependant though, and we have a cruising guide available covering that area. It is also available off our website www.cruisingconnections.co.za
The above is the most direct route one could do. Your other option would be via the Mozambique Channel and the Seychelles etc. If you want any info on this route I'll be happy to assist.