This particular Gratiola species behaves more like a ruderal plant than a water plant: my observations of this species have not suggested it has any adaptations to living in an aquatic environment or that is life cycle is dependent on water regime. Rather it seems to flourish on poor soils; in the absence of competition; and be very plastic in how it grows. for example, it is still present at this site over he last few months (noted in Nov 2019 and Jan 2020), very stunted but reproducing, and about 2 m above the water level in the dam.
ALA shows that Rosemary Purdie collected a specimen from here in 2014. ALA includes CNM photographs but instead of showing Gratiola pedunculata, it selected the one showing the species habitat so shows a line of Juncus.