[citation needed] He realised the importance of endangered species early in his career when, in 1933, he was the last person to photograph a captive thylacine or Tasmanian tiger at the Hobart Zoo.

Later that year, disagreements with the zoo's management came to a head and Fleay was dismissed, principally because of his belief that native birds and animals should be fed what they would eat in the wild.

[citation needed] In 1962 Fleay co-founded the Wildlife Preservation Society of Queensland with Judith Wright, Brian Clouston and Kathleen McArthur.

The animals there included quolls, Tasmanian devils, dingoes and various birds of prey, to which he added tiger snakes that were milked for antivenene, and platypus.

[citation needed] Between 1945–1947, Fleay led an expedition to Tasmania in an attempt to capture a breeding pair of thylacines; however, he returned empty handed.

[citation needed] After extensive research, the Tallebudgera Estuary in the hinterland behind Burleigh on the Gold Coast in Queensland was selected, the reasons including that it offered an untouched natural habitat for koalas apart from already having cleared areas (then farmland) for development of animal enclosures.

The Fleays gradually acquired land and by 1958 had enclosures for people to see platypuses, snakes, dingoes, plain turkeys, ospreys, crocodiles and alligators; in contrast, bandicoots, flying foxes, sea eagles, wallabies and koalas, were free to visit from adjoining the forest.

[citation needed] The animals were fed partly from donations from local bakers and butchers, with local residents donating dead animals to feed the owls (or the goannas if no longer fresh); mice and rats were collected frequently from the McKerras Research Institute behind the hospital; worms were collected fresh daily for the platypuses; eels, pigeons and flying foxes were also killed to provide food for the owls, snakes and crocodiles.

[citation needed] In 1982, 37 acres (150,000 m²) of the land owned by David and Sigrid Fleay was sold to the Queensland Government and became a Conservation Park.