Photographed at Mountain Zebra NP.
Although I cannot see the picture I seriously doubt that the spider is Nephila inaurata simply because of the locality. It's probably Nephila senegalensis, just like all of the other Nephila in this thread.
N. inaurata occurs more towards the East Coast of southern Africa all the way up north. It's a tropical species occurring in moist humid vegetated areas. The information on N. inaurata is pretty much the same for most members of Nephila. Good Stuff!
Something that is cool to do with Nephila. I was at Singita Lebombo late in the spider season.... May I think. Because it was the end of their annual lives the females were laying their egg sacs, then dying. If you ever see an empty, broken web with no spider, follow the supporting lines up to the nearest bush, tree of bit of vegetation. You'll often find a ball of loopy white silk. This is the spiders egg sac.
If you look at their webs towards the end of the spider year (Dogs have Dogs year, so Spiders can have Spider years). Look in webs and you'll see the female (hard to miss) and much smaller spiders (often with legs missing) (Shame) around the periphery of the web. These are the male spiders lurking in wait for the right time to sneak down and mate. Disturb the female and she usually retreats to an upper most corner of her web. Disturb a male and he usually drops out of the web on a strand of silk. The female is not of medical importance even though they can give a nasty bite that leaves a small scar.
You'll also find Argyrodes sp. spider (Dew drop spiders) in the females web. These spider feed from what the female has caught. They are silvery in colouration and look like "drops of dew" or mercury (not the planet, but the metal). There are also "spider flies" that are known to hang around these webs. These flies land on the spiders mouth parts while she is feeding. The flies then suck the liquid that is produced while the spider is feeding. Much like a fly would suck drool out of our mouths when we are eating an exceptionally yummie slice of chocolate cake.
(You get the point...)
The yellow silk is not really special, it is just synthesised out of silk glands that produce yellow silk. The webs are know to catch small birds and bats.
Next time you see one of these webs have a closer look. You may just find more than you expect...