Eucalyptus petrensis is a straggly mallee that typically grows to a height of and forms a
lignotuber. It has smooth, light grey bark that is shed in strips, sometimes with rough bark at the base of the trunk. Young plants and
coppice regrowth have dull green, egg-shaped leaves that are long and wide. Adult leaves are the same shade of glossy green on both sides, lance-shaped, long and wide, tapering to a
petiole long. The flower buds are arranged in leaf
axils in groups of between seven and thirteen on an unbranched
peduncle long, the individual buds on
pedicels long. Mature buds are long and wide with a beaked
operculum that is two or three times as long as the
floral cup. Flowering occurs from June to July or October and the flowers are creamy white. The fruit is a woody, more or less spherical
capsule long and wide with the valves initially protruding but fragile. The mallee has a similar appearance to
Eucalyptus foecunda but
E. petrensis has a more persistent style on the fruit. ==Taxonomy and naming==