I enjoy IDing gerrids because there are some good resources available. For this one I primarily used this paper. Genus is quite easy - the first antennal segment is distinctly shorter than the second and third combined (ruling out Aquarius), and the pronotum is dark and shiny with a pale central stripe (ruling out Limnometra and Tenagogerris).

We have four Limnogonus species in Australia and it seems all except L. windi are within range here. The key indicates that L. windi is a paler, more robust species though so I think we can rule it out for good either way. Next we look at the pale mesopleural stripe. It widens posteriorly (ruling out L. fossarum gilguy) and it does not enclose the metathoracic spiracle (ruling out L. hungerfordi), so that would make it L. luctuosus. Excellent!