Significant sexual dimorphism exists between the males and the females of this species, with males growing to be larger. The maximum recorded weight in males is 9.1 kg, while in females it is 6.9 kg. The body length is 59 to 68 cm in males and 52 to 63 cm in females. Both males and females are about 45 cm in height. The tails of males range from 38 to 45 cm and that of females from 33 to 44 cm. They have a greyish brown pelage and reddish shoulders. As with most wallabies, the hind quarters are powerful and well-developed and the strong, tapered tail is used for balance. The tail acts as rudder when in motion and as a third leg while seated, being strong enough to support the full weight of the animal.

M. eugenii is herbivorous, foraging in open forests or savannahs and relying on dense vegetation for cover. Tammars are primarily nocturnal and crepuscular, resting during the daytime. This wallaby is found in areas with almost no fresh water. It obtains what it needs from salty plants and can even drink sea water. The social structure of M. eugenii is not solitary, but it is not a tightly organized group either.

The average length of estrous for M. eugenii is 28.4 days. All females of Macropus are polyestrous, but M. eugenii has a specific breeding season from January to June or August, and most births occur from mid-January to mid-February. Females have a forward pouch and four mammae. Only one young can be produced each year. The nondelayed average gestation period is 28.3 days. As in other species of Macropus, embryonic diapause occurs, allowing a dormant blastocyst to develop if the first young ceases to suckle in the pouch. In M. eugenii, if the young is lost while the breeding season is in progress, a subsequent birth occurs 27 days later. If, however, the original young survives, the blastocyst remains dormant until the following breeding season, approximately 11 months after fertilization.

M. eugenii is found in SW Western Australia, South Australia, Kangaroo Isl, Wallaby Isl and other islands.