I'm pretty sure it is Oxalis drummodii, but I don't know why it's blooming in fall since it's a spring bloomer. Check out this link.

Here is a link that might be useful: Oxalis drummondii

I suspect that it may be an introduced species since many of them bloom throughout the growing season. The flowers in the pic look a little like Oxalis debilis.

Here is a link that might be useful: Oxalis debilis

What you have is Pink Wood Sorrel. I have it in my flower beds. It blooms throughout the year except during very cold weather.

well, there's nothing like making a fool of yourself world-wide. What I MEANT to say in the message above is that you have OXALIS CRASSIPES, common name Pink Wood Sorrel. I have it in my flower beds & it blooms year round for me. I have the ALBA (white)blooming variety: common name White Wood Sorrel. It's not as tough as the pink. The leaves are also different; they aren't rounded on the edge like the pink. They look like the round edge has been cut straight across with scissors. I love both types.

Here is a link that might be useful: white wood sorrel