An unexpected find.
About five plants scattered along short part of creek-line.
Plant is showing autumn senescence. The inflorescence (just visible in third photo) was all dead and at seeding/fruiting stge.
From the nearby trail, foliage looked like Sagittaria which was what prompted having a closer look.
I first thought this might be a remnant of the 2005 Friends of Mt Majura plantings at the twin dams (we lost most plantings unfortunately) however, the list of the planted species I have from that time doesn't have Water plantain.