The case was around 4 cm, maybe certainly not very large. Caterpillar was pretty energetic except when I placed it (gently) on the track for a better photo at which point it refused to come out until I put it back in the grass.
Sorry, it was on the stockyard spur management trail off Mt Franklin Road in the Brindabellas so we left it where we found it. The trail is quite grassy between the wheel tracks around there possibly poa but I'm not sure.