Now, that's a fairly large tree - I'd guess it's in excess of 100 years old. So, either someone drilled a horizontal hole into the tree and then a vertical one straight down into the ground (which seems unlikely to me), or the water point is so old that the tree grew around the original pipe.
In any event, it was kinda weird to see a tap coming out of a tree with no obvious pipes in the vicinity.
See this post for more about this photo and Rustenberg Wine's open gardens.