These photos were taken over two years ago, on the same day I photographed the sea centipedes. I wasn’t able to identify the species until recently.
I had no idea where to start with this critter. I put it on iNaturalist and fortunately, somebody was able to narrow it down to the Sphaeromatidae family. Normally family alone wouldn’t be enough for me to go on. However, my trusty ‘Some intertidal Isopods of Victoria’ booklet saved me again.
I decided that it was because the body was flat and the pleotelson ended with a sharply pointed spine.