It is almost impossible to ID this group of Oxalis fron a photo like this. I would need field notes (or good photos where possible) on whether the stems are rooting at the nodes, the number of flowers in a cluster, the hairs or lack thereof on the stems, and the shape and size of the stipules at the base of the leaf stalk, or the lack of stipules. The upright fattish seed cases suggest exilis rather than corniculata, which has narrow seed cases on stalks which bend down, but I am not sure whether that is enough to say that it is not one of the other native species of Oxalis.