From a suggestion from Snails of Australia facebook page, I think this is Onchidina australis, Air-breathing sea slug. https://bie.ala.org.au/species/https://biodiversity.org.au/afd/taxa/15abc931-702a-4053-a607-b56175aa1ce0 Habitat seems to be muddy tidal areas such as Mangroves and salt marshes. These were in a muddy tidal creek.
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