N. olivacea is a small snake which exhibits sexual dimorphism.

Males may attain a maximum total length (including tail) of only 27 cm (11 in), but the larger females may attain a total length of 33 cm (13 in).

The upper labials are whitish, with dark vertical bars at the sutures.

[3] N. olivacea is found in Angola, Benin, Botswana, Cameroon, Central African Republic, both Congos (Democratic Republic of the Congo and Republic of the Congo), Equatorial Guinea, Ethiopia, Gabon, Ghana, Guinea, Ivory Coast, Mali, Mozambique, Namibia, Nigeria, Senegal, Somalia, South Africa, Sudan, Tanzania, Togo, Uganda, Zambia, and Zimbabwe.

In early summer the adult female lays a clutch of 6-8 eggs.