Measuring 8–10 cm in length, this unassuming thornbill is characterised by its plain greenish brown upperparts and very pale-yellow underparts, with a distinctive buff coloured rump.

Early classifications of the genus Acanthiza based on plumage and morphology recognised one to three genera with a varying number of species in each.

[4] The thornbill's diet consists primarily of small insects and plant lice gleaned from foliage.

[8] A. reguloides builds a large, dome shaped nest consisting of bark strips, dried grass and moss located one to two metres above the ground.

[9][8] Females typically lay clutches of 2-4 eggs in late August with an incubation period of 20 days.

Egg hatching is highly synchronous and adults of the cooperative breeding group work together to feed the chicks.

[9] Buff-rumped thornbills are often found in small, chatty groups of approximately 20 individuals,[8] but may join during non-breeding season to form larger clans.