Hi Tapirlord, Juncus are notoriously difficult to ID, so I'd love to know how you ID'd this juncus?? One of my books says that it is not in the Southern tablelands, although in this spot it could easily have been introduced. i believe the recent Austrostipa verticillate was planted by the Coolo group many years ago. I have it in my garden and it is a bit weedy, but the one of the dam is not spreading much.
Yep, Juncus are certainly notorious! In terms of the ID, I'm going off the shape and structure of the inflorescence and the size and colour of the culms. The cataphylls being red also helps. This isn't a great way to ID anything in this genus generally, but I have spent a good amount of time working with this genus (Identifying them properly with pressed specimens) and so I am pretty confident it will be this species; it is the most common species in this kind of habitat and at this size. J.vaginatus is quite common in the ACT, and definitely is local. It has done well these last couple of years with all the rain.