I’ve discovered there are two species of yellow flowering Wood-Sorrel (Oxalis) which look very similar, and are often confused. One is native to Australia, the other is an exotic (weed) originally from South Africa.
Looking at the flowers of both species, I believe this could be the Australian native plant Grassland Wood-sorrel (Oxalis perennans). Can anyone confirm this? For reference purposes, the exotic species is Yellow Wood-sorrel (Oxalis corniculata).
Fifteen Acres 
Hey, this is an extremely delayed reply to your questions, but the Oxalis species are extremely difficult to differentiate without a number of features. You probably have more than 2 yellow-flowered species of Oxalis. Here are some ways of telling:
Grassland Wood-sorrel (Oxalis perennans) stems hairs point upwards, fruits are 8-30 mm long, narrow (more than 7 times as long as wide) and held above the leaves, stipules are conspicuous with rounded tips and taproots are stout.
Creeping Wood-sorrel (Oxalis corniculata) stem hairs are spreading, fruits are 8-16 mm long, not slender (less than 7 times as long as wide) and not held above the leaves, stipules are conspicuous and truncate and taproots (if present) are slender.
Shade Wood-sorrel (Oxalis exilis) stem hairs are sparse and mostly point upwards, fruits are 5-6.5 mm long, not slender (less than 7 times as long as wide) and held above the leaves, stipules are conspicuous and truncate and roots are fibrous.
Tap-rooted Wood-sorrel (Oxalis radicosa) stem hairs are spreading or point downwards, fruits are 7-20 mm long, not slender (less than 7 times as long as wide), stipules are inconspicuous and taproots are stout.
Thanks Harry, and a delayed response from me to you as well… thanks so much for this information. I think it’s timely that I’m reading it now as they are starting to flower again. This time I will know what to photograph.
I really appreciate you taking the time to share all of this information with me. 😊