This is a very good sighting. Hopefully we'll get a Gastropoda expert before too long. It is similar to the Otway Black Snail, Victaphanta compacta, see: https://alisonpouliot.com/colac-otways-shire/otway-black-snail-victaphanta-compacta-6956/
There are several species of Victaphanta listed on ALA: https://bie.ala.org.au/species/urn:lsid:biodiversity.org.au:afd.taxon:93e29b6f-2c4a-442a-8276-6b4003740b61 and https://bie.ala.org.au/species/urn:lsid:biodiversity.org.au:afd.taxon:93e29b6f-2c4a-442a-8276-6b4003740b61#classification
Perhaps we have a more local species... suggestions welcome.
Wondering if this could be a semi-slug, something like Helicarion masteri? That species has been found in Namadgi before? I think there is also at least one species of undescribed semi-slug in the Brindabella's.
Thanks Greggy I've added Helicarion mastersi to our taxonomy since it has been recorded locally: https://bie.ala.org.au/species/urn:lsid:biodiversity.org.au:afd.taxon:c13fcca1-2839-448e-a644-18e1e4047f63
This snail was kindly identified as H. cuvieri by Michael Shea via the facebook group "Snails of Australia" and co-author of the field guide "Australian Land snails". Absolutely beautiful species, I hope to see more of these on CNM.