There are two forms of extinction. The first is where a population is below genetic viability and the second where the total number of animals is known to be 0. Australia is set up to account for these two forms with an extinction in the wild status and then a formal extinction classification. The use of these status was best illustrated with the extinction of the Thylacine.
The last known specimen died in captivity in Hobart on the 7th of September 1936. From there it was classified as extinct in the wild. On the 7th of September 1986 it was declared extinct. This may be where the "presumably" extinct idea for Thylacine is derived from despite its official extinct status.
Extinct species do get re-discovered, see the article below which gives us 14 reasons to hope for the Thylacine. Which was brought to my attention by Bryce Pinson from the www.facebook.com/thylacineresearchunit page;
Australia has several examples of mammal species, that have been rediscovered. For instance the Gilbert's Potoroo which First discovered in 1840 by naturalist John Gilbert and recorded again only a few times over the next 40 years, this small rat-kangaroo was thought to be extinct since the late 1870s until rediscovered in 1994 at Two Peoples Bay near Albany, on the South Coast of Western Australia.
There are thought to be only 30-40 animals in the wild and 40-50 animals in captivity and is critically endangered and is Australia's rarest Mammal.
Or The Mountain Pygmy Possum was first described as a Pleistocene fossil by Robert Broom in 1896. It was thought to be extinct until 1966, when a living specimen was discovered in a ski-hut on Mount Hotham (Wikipedia)
Or if your concerned, as I was, that something that small could easily go undetected try the Bridled Nail Tailed Wallaby.
The last reported sighting of a Bridled Nail Tailed Wallaby was in 1937. Then, in 1973, the animal was reported on a cattle station near the town of Dingo, Central Queensland by a fencing contractor. Males are up to 8 kg in size (www.bntwallaby.org.au) not a 32kg Thylacine, granted, but it sets a precedent as a rediscovered medium sized mammal, in Australia.
Or what about an iconic animal like Victoria's state emblem the Lead Beaters Possum
The possum was discovered in 1867 and was originally known only through five specimens, the last one collected in 1909. From that time on, the fear that it might be extinct gradually grew into near-certainty after the swamps and wetlands in Australia around Bass River in south-west Gippsland were drained for farming in the early 1900s. Then, on 3rd April 1961, a member of the species was rediscovered by naturalist Eric Wilkinson, and the first specimen in more than 50 years was captured later in the month (Wikipedia)
In a recent article in Australian Geographic Dr Diana Fisher was quoted, when questioned about Bridled Nail Tailed Wallabies as saying "I think it might be useful to know that rediscoveries are not random, "If we know which types of species are most likely to be alive but hard to detect we might be able to better target searches for missing species." She also stated in Australian Geographic that "Many species feared extinct from habitat loss have in fact turned up in a different habitat, because they were more flexible than we realised," says Diana. "Some species can also persist in marginal habitat for some time." (Australian Geographic Magazine)
So despite recent reports of Thylacine having such limited genetic variance that they are 99.5% similar, the persecution of Thylacine as a sheep killer under bounty systems, continued habitat loss, genetic fragmentation of populations through habitat loss, island isolation, genetic bottle necking and a myriad of other issues one would like to have hope that the thylacine like these species could still be hanging on. Maybe not in genetically viable numbers but hanging on all the same and like these rediscovered species above, managed through programs to ensure their survival!