There's an
Acacia and Mimosa identification thread with ca 70 pages, maybe sift through that if you have time and feel like it. Or maybe a mod will move this there.
I don't know enough about Acacias to make any good guesses. At first glance i noticed the phyllodes looking slightly similar to the ones on my supposed Acacia acuminata (long and with curved tip, but rather too wide), but i think the flowers are wrong and i never heard of it growing in South Africa (which doesn't say anything). I live in Switzerland, so i don't encounter any Acacias in the wild - just happy that i got them to sprout and not die on me yet, and hoping the seeds were labelled correctly
So i tried some searching. One i found that looks very similar to my eye is
Acacia saligna, of Australian origin too but also cultivated on different continents. That site has a list of identification keys, the only one listed for Africa is
http://worldwidewattle.com/infogallery/identification/ wrote:Ross, J. (1979). A conspectus of the African Acacia species. Memoirs of the Botanical Survey of South Africa 44: 1–155.
Maybe you can find it in a library or online...
Can you tell a bit more about these trees? How tall do they grow, what environment are they in, like soil type, wet/dry, coast or inland, etc? A picture with the entire tree might help too.
I hope someone who actually knows things about Acacias will chime in and solve the mystery...