Diprotodon species fossils have been found in sites across mainland Australia, including complete skulls and skeletons, as well as hair and foot impressions. Female skeletons have been found with babies located where the mother's pouch would have been. The largest specimens were hippopotamus-sized: about 3 metres (10 ft) from nose to tail, standing 2 metres (6.6 ft) tall at the shoulder and weighing about 2,800 kilograms (6,200 lb). Aboriginal rock art images in Quinkan traditional country (Queensland, Australia) have been claimed to depict diprotodonts. They inhabited open forest, woodlands, and grasslands, possibly staying close to water, and eating leaves, shrubs, and some grasses.
The closest surviving relatives of Diprotodon are the wombats and the koala. It is suggested that diprotodonts may have been an inspiration for the legends of the bunyip, as some Aboriginal tribes identify Diprotodon bones as those of "bunyips".
The first recorded Diprotodon remains were discovered in a cave near Wellington in New South Wales in the early 1830s by Major Thomas Mitchell who sent them to England for study by Sir Richard Owen. In the 1840s Ludwig Leichhardt discovered many Diprotodon bones eroding from the banks of creeks in the Darling Downs of Queensland and when reporting the find to Owen commented that the remains were so well preserved he expected to find living examples in the then unexplored central regions of Australia.
The majority of fossil finds are of demographic groups indicative of diprotodonts dying in drought conditions. For example, hundreds of individuals were found in Lake Callabonna with well-preserved lower bodies but crushed and distorted heads. It is theorised several family groups sank in mud while crossing the drying lake bed. Other finds consist of age groupings of young or old animals which are first to die during a drought.
In 2012, a significant group of about 40 was found at Eulo, South-West Queensland.
Diprotodon was named by Owen (1838). It was assigned to Diprotodontidae by McKenna and Bell (1997). The historical classification of Diprotodon consisted of eight species (Diprotodon optatum Owen, 1838; Diprotodon australis Owen, 1844; D. annextans McCoy, 1861; D. minor Huxley, 1862; D. longiceps McCoy 1865; D. loderi Krefft, 1873a; D. bennettii Krefft, 1873b (nec D. bennettii Owen, 1877); and D. bennettii Owen, 1877 (nec D. bennettii Krefft, 1873b); based on size or slight morphological differences of single specimens collected from isolated geographic regions. Bimodal dental sizes, rather than a continuum of tooth sizes, and identical male and female dental morphology, indicate sexual dimorphism instead of separate species, thus providing strong evidence that the eight species are synonyms for D. optatum.