This is a Flatworm, Caenoplana bicolor, Two-tone Planarian. See https://bie.ala.org.au/species/urn:lsid:biodiversity.org.au:afd.taxon:1b49d27f-a60b-433f-8d4a-debbc22ceac9 and https://canberra.naturemapr.org/species/46288 I noticed you don't have a flatworm category, but you might need to make one, and more flatworms are sure to pop up. Cheers.