The photo doesn't provide quite enough information to be sure on this ID. The culms of this species can be compressed, but are usually terete or somewhat compressed.
Mike this and Jerrabomberra wetlands are the only place that any Baumea has been recorded in the ACT and this East O,Malley pond has two species - which makes me suspect that they were planted - what do you think?
The pond was constructed and filled in 2005. It would be good to get a planting list - several shrubs and trees are inappropriate. It is likely the larger rushes and sedges have spread from plantings.