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Anguilla australis
~1.5m eel, blue-grey with dark stripe to sides of face, and rainbow effect sheen on tail side frills. Small rounded pectoral fins.
Fresh water ponds and creeks.
Neil has indicated they are likely to be Aguilla reinhardtii or Speckled Longfin Eels, as identified at the Uni. I have no idea, other than what I can gather from google, but they certainly were the ones in the lake!