[8][9] They differ in extent of reddish-brown and intensity of streaking, as well as size:[10] The related Chatham fernbird (Poodytes rufescens), which became extinct around 1900, was sometimes treated as a subspecies of this species.

[14] The New Zealand fernbird is a ground-dwelling bird, and is a reluctant flier, travelling mainly on foot or in occasional short flights of less than 15 metres.

In the 19th century, Walter Buller described it as "one of our most common"[14] birds, but it has been adversely affected by the subsequent widespread destruction of its natural wetland habitat following European settlement and is now rare.

[14] The New Zealand fernbird is present in Oceania, mainly in wet shrubby environments with swamps, peat bogs or ponds, plantation areas, temperate shrublands.

[16] Field studies of the South Island fernbird showed they preferred shrubby habitats with low dense ground vegetation near an estuary margin.

The contact between the two partners is a duet, a characteristic "u-tick",[9] that is, a sonorous "uu" for the male, to which the female answers with a high-pitched "tick".

[18][19] The birds nest in sedges or other vegetation close to the ground, making a deep woven cup of dried rushes lined with feathers.

Breeding occurs from September to February, producing clutches of 2–3 pinkish-white eggs with brown or purple speckles.

[21] In a two-year field study, South Island fernbirds strongly defended their territories during the preebreeding and breeding seasons.

[14] The calls of the bird were interpreted as heralding success or failure in daily activities such as fishing, but on a more serious level they could also portend prosperity and health or disaster and death.