This sunbird is common in gardens, fynbos, forests and coastal scrub. The southern double-collared sunbird breeds from April to December, depending on region. The closed oval nest is constructed from grass, lichen and other plant material, bound together with spider webs. It has a side entrance which sometimes has a porch, and is lined with wool, plant down and feathers.

The southern double-collared sunbird is usually seen singly or in small groups. Its flight is fast and direct on short wings. It lives mainly on nectar from flowers, but takes some fruit, and, especially when feeding young, insects and spiders. It can take nectar by hovering like a hummingbird, but usually perches to feed most of the time.

The call is a hard chee-chee, and the song is high pitched jumble of tinkling notes, rising and falling in pitch and tempo for 3–5 seconds or more.