its difficult to sort this out. The location looks more like a soak than a creek, and if so then it might not be a water plant. There appear to be some purple flowers but not distinct enough to ID, possibly a Veronica ? Some of the leaves resemble Nasturtium officinale (which has white flowers). A height of 50 cm is on the tall side for N. officinale
This was definately in a small creek, but I did struggle as to where to put this plant, it may well belong in another plant category. There were no flowers on it at all, so that doesn't help much sorry.
Sorry I can't resolve this. Hopefully another moderator can.
The reason I thought it looked like a soak rather than a creek was because of the photo location, which is up high above Ginninderra Creek on uniform-looking ground.