At "less than 5mm", this cannot be Eretes australis, which is 9-15mm. The two species also differ in their markings, and in the outline of the pronotum and elytra.
Sigara (Tropocorixa) sp. in the 'Water boatmen' family, Corixidae . ALA has records of two species in your district: S. (T.) neboissi and S. (T.) sublaevifrons, but I don't know which this is. btw Tropocorixa is a subgenus within genus Sigara
