The Chatham Island warbler has a plain olive-brown head and upperparts, with off-white underparts interrupted by pale yellow flanks and undertail. The male warbler has a distinctive white forehead, eyebrows, throat and underparts. The female warbler lacks these white areas, instead showing dull greyish-white underparts and yellow eyebrows, cheek and throat. Both adults have red eyes. This species of warbler also shows variation in the plumage of the juvenile bird. More akin to the female colouration, the juveniles have olive-grey upperparts and more yellow through their underparts, with brown eyes.