Thank you very much to Denis Riek, an incredibly knowledgable naturalist and nudibranch enthusiast living in Byron Bay, for correcting my identification of this slug from last week. It's Dendrodoris carbunculosa, not Hexabranchus sanguineus.
Denis is a regular contributor to the Sea Slug Forum, Nudi Pixel, and nudibranchs.com.au. Check out Denis' website at www.roboastra.com.