25mm length
Identified as Calosoma oceanicum the darker caterpillar eating carabid by Nadine Guthrie on Western Australian Insects FB group.
It was found apparently drowned but after an hour out of water, it revived!
6 limbs with setae; long antennae. Mandibles present. Shiny, hard body segmented into three parts. Body is flat and broadly shaped.
Found under rock in suburban backyard.
Found in pool
About 4cm in body length
Maybe: Blackclock Ground Beetles Genus Pterostichus