Not sure about this one as I know there are a couple similar ones but hopefully not sagittaria. Each leaf was up to about 1 meter tall and the flowers had three white petals and were the same height as the leaves. I’m happy to return any time and take better photos as all I had was my iPhone from some distance.
Well spotted. It certainly looks like Sagittaria platyphylla, and it looks to be very well established in this urban wetland and may be in downstream places. Close ups of flowers and fruits would be a help - is this possible at all ? or an observation. Sagittaria has numerous stamens, and flowers mostly uni-sexual: whereas Alisma (same family, very similar) has six stamens and flowers mostly bi-sexual.
Hi Jane, I'll return to the site today and try to get some close-up photos of the flowers and fruits. The first photo is of a small number next to Clunies Ross Street and the last photo is of many plants that were in the main pond just downstream. I couldn't see any further down from that but the creek feeds directly into Sullivan's creek so it may have already been established in that watercourse.
@JaneR I took some pictures of the flowers and fruit so hopefully that helps. Sorry that they're still not great but the constant stream of European Wasps flying near the base dissuaded me from getting too close.