Wow. That looks like dayflower - the stuff that is all over my containers now. LOL I never planted it but managed to get "infested" with it when I dug up some extra perennials from one of my sister's yard and potted them up.

Here it was (with the little blue flowers, although it's hard to see its leaves) a couple years ago with a petunia volunteer! The leaves for it look identical to what you have. It blooms here in mid-late summer and the flowers only last a day - but by golly the seeds go flying every which way including up because I've had sprouts popping up in windowboxes!

Here is a link that might be useful: Commelina communis - Asiatic dayflower

No, it's not dayflower (Commelina) but a species of Tradescantia, and you're right, it's one of our worst garden weeds in subtropical eastern Australia. It has been variously named T. fluminensis and T. albiflora but the truth is that its correct identification is still in question. It was usually known as "wandering jew", though many bushland weeders now prefer to call it "trad"