HI Kathryn, it does self-seed but is prone to a whole range of animals that keep it in check. From the NZPCN
PROPAGATION TECHNIQUE
Easily grown from seed, semi-hardwood cuttings, and stem layerings. Plants tend to be short-lived in cultivation (2-4 years), and benefit from hard pruning after flowering. Kaka beak is vulnerable to a range of common garden pests which include slugs and snails, it can be severely defoliated, by these animals, and young plants may be killed completely. Caterpillars, mites - which cause witches brooms, and various fungal diseases will also kill plants. To combat these problems grow plants in fertile, well drained, sunny sites free from surrounding shrubs. Despite its northerly distribution, kaka beak often does best in Southland, and is very tolerant of snowfall, and light frosts.