South Island Fernbirds near Invercargill, Southland, were studied for two years. Adult birds had year-round preference for areas with low dense ground vegetation and emergent shrubs. The preferred habitat occurred in a linear strip along an estuary margin. Territories were contiguous and distributed in linear fashion along the strip. Territory areas varied from 530 m2 approximately to 2870 m2 approximately. Territories were strongly defended during the prebreeding and breeding seasons, and defended to some degree at other times. Juveniles were often found outside the preferred adult habitat. There were indications that total Fernbird population needs may include an area adjacent to but exclusive of adult territory areas. Fire had a prolonged adverse effect on breeding habitat.