yes, i was a bit surprised to see it, especially as I am 90%sure it wasn’t there 2 years ago. there is a drain not far away in Duntroon that has quite a bit of Cx fascicularis beside it, quite possibly planted as an ornamental. Its possible that this supposed planting is a seed source though it’s a puzzle as to how it might be dispersed.
I have it on my dam and associated creekline in Wamboin. Generally I find the flora here pretty similar to Majura so possible it's natural though who knows
I found it in the local suburban pond in Watson last year, not likely to have been planted where it was growing, probably seed carried in mud on the feet of birds.