The fat-tailed dunnart has a head and body length of 60-90mm, tail 45-70cm in length and weighs only 10-20g. It has large black eyes, large ears, a pointed snout and a fat tail when in optimum condition. Most of the upper body is fawn to brownish grey in colour, with darker patches around the eyes and head. Some individuals have white crescent shaped patches around the ears. The underside and the legs are usually light grey to white in colour.
Fat-tailed dunnarts occupy a variety of open habitats, including open woodland, low shrublands and arid shrublands. Populations can also be found living in areas of agricultural land such as unimproved pasture, they have been found in old hay sheds, amongst rock piles and old logs. Their distribution is west of the Great Dividing Range, south of the Tropic of Capricorn, south of the Kimberley region in Western Australia and most of South Australia.
The fat-tailed dunnart is nocturnal and mainly feeds on small invertebrates such as beetles, spiders, slaters, worms and slugs. Body fat is stored in the tail when food is plentiful, giving the tail a swollen appearance. This helps the individual to survive when food is in short supply. The tail generally appears thinner in winter when food is scarcer. If food is very scarce, usually in the cooler months, dunnarts may enter a deep temporary sleep known as torpor.
Fat-tailed dunnarts are short lived, about 15 months for males and 18 months for females which means successful breeding is important for populations to survive. Breeding occurs between July and February. Females construct nests under logs, rocks, or in deep cracks in the ground, made from dried plant material. Adult fat-tailed dunnarts have multiple partners, but most individuals nest alone during mating season. Usually 8 to 10 young are born about 13 days after mating. The young attach firmly to the 8 to 10 teats that the female has in her well-developed pouch. They leave her pouch permanently approximately 60 days after birth, staying in the nest while the mother forages for food.