Oxalis (sour sob) is a native of South Africa. Some kind soul must have thought it was pretty and introduced it to Australia. It is highly invasive and has been declared a noxious weed.
My poor old dad had it in his vegetable garden. It came in a pot with a 'legitimate' plant. He spent many heartbreaking years trying to deal with it.
Cultivation only encourages it. It grows by bulb and seed. If you weed it by hand, the stalks and leaves break off and it pops up again. Dad sifted his entire vegetable garden several times. But some of the bulblets are tiny and it is impossible to find them all. Even when heavily mulched, its stalks will make their way to the surface. If it is allowed to flower it seeds profusely.
Gardening Australia suggest feeding it to death. But the last line in their advice is. "This can take several years of repeated treatments to be totally effective". Did you read that carefully...SEVERAL YEARS! In other forums people suggest weeding on a daily basis to weaken the bulbs. Others recommend covering it with black plastic to 'cook' it. My experience is that it will just lay dormant until the right conditions occur and reappear.
The only way I have ever been able to beat it is to poison it. I paint the leaves with the herbicide...I don't spray. It will diligence and several applications to finish it off. This is feasible because I only have three small patches. I think I would sell the house and garden if there was any more!
Sorry to all the purists out there but I can't let it get a hold.