Tim Leach, I would like to post these images on the 'Snails of Australia' Facebook group if you don't mind. I have a feeling this species of Cystopelta slug is not native to the ACT area. It more resembles a species from northern NSW. Is that OK with you ?
This species may be Cystopelta septentrionalis Stanisic, 2010 Family Cystopeltidae. This native species occurs on the north coast of NSW reaching into the far SE corner of Queensland (Lamington NP and the Scenic Rim).