What a wonderful find. Any chance of rearing it in captivity to get an adult moth to identify (then you could take it back and release it where you found it)?
Yes, there are plenty of opportunities to contribute to science re aquatic moth larvae and aquatic moths generally. A colleague provided me with this link to a paper which includes some useful overview information regarding ing the state of knowledge and a key to Nymphulinae (Lepidoptera: Pyralidae) associated with Hydrilla verticillata (Hydrocharitaceae) in North Queensland. file:///C:/Users/Andrew/AppData/Local/Microsoft/Windows/INetCache/Content.Outlook/QE97E9MU/Habeck%20and%20Balciuns%202005%20aquatic%20moth%20caterpillar%20Hydrilla%20Australia.pdf
Ianm: that is an address on Andrew's own computer. We cannot access it. The abstract in question is at https://www.researchgate.net/publication/262871386_Larvae_of_Nymphulinae_Lepidoptera_Pyralidae_associated_with_Hydrilla_verticillata_Hydrocharitaceae_in_North_Queensland but there are lots of articles popped up by google if you just search aquatic caterpillars australia The commonest species is Hygraula nitens, but the larva of that species is reported to live in a case, so these photos may be of a related species.