Nudged it (with a twig) and it was firm and unmoving (and definitely not a rock, like I had first thought).
Oddly split, or two merged? ...located under a grevillea plant.
Related Observations:
Plant - https://inaturalist.ala.org.au/observations/210075436
Insect - https://inaturalist.ala.org.au/observations/210075440