Opossum Bay was almost certainly named after the American sloop ‘Opossum’ which was chartered by the government under Harbour-Master Capt. John Welsh to conduct numerous surveys. He surveyed the River Derwent and River Huon in 1827 and Port Arthur later.
Seemingly the Opossum anchored in Opossum Bay whilst the ship’s boats conducted surveys up the river. This certainly happened at Surveyors Bay at the mouth of the Huon River during surveys of that river. He also had a land grant at South Arm in 1822 which he had abandoned by 1823.
Also of interest is that Opossum was the rescuing vessel for the castaways thrown ashore by the convicts who took over the "Cyprus" at Recherche Bay in 1829. The government-owned brig 'Cyprus' was on its way to the penal settlement of Macquarie Harbour carrying 31 convicts under sentence of transportation and 31 other people. During a stop in Recherche Bay, a number of prisoners mutinied, took possession of the vessel and carried her out to sea. Lieutenant Governor Arthur ordered the Government-owned sloop Opossum to Recherche Bay to rescue those put on shore by the mutineers. Read more at https://eprints.utas.edu.au/13917/1/1954_Pretyman_Pirates_at_Recherche_Bay.pdf
Question: was his grant at Opossum Bay? If so, his ship would almost certainly have anchored in the bay whilst he was there. He named a bay in Port Arthur, Opossum Bay (now Carnarvon Bay) and he also named Opossum Island in that region. Welsh was Harbour Master at Hobart in the early 1820s and later at Launceston c1829. He died in Launceston in 1832.
Thanks to Wayne Smith for this information.