It has smooth bark, lance-shaped adult leaves, flower buds in groups of seven, yellowish green flowers and hemispherical fruit with the valves extended well beyond the level of the rim.Eucalyptus extensa is a mallet that typically grows to a height of 3–8 m (9.8–26.2 ft) but does not form a lignotuber.

[2][3][4] Eucalyptus extensa was first formally described in 1991 by the botanists Lawrence Alexander Sidney Johnson and Ken Hill in the journal Telopea, from a specimen collected near the road between Hyden and Norseman.

[2][5] The specific epithet is taken from the Latin word extensus meaning "stretched out or extended" in reference to the long opercula on the buds.

[3] This mallet occurs on sandplains and undulating areas along the south of Western Australia in the southern Wheatbelt and south western Goldfields-Esperance regions where it grows in red loam, grey sandy loam and sometimes gravelly soils.

[4] This eucalypt is classified as "not threatened" by the Western Australian Government Department of Parks and Wildlife.