About half of New Zealand's c. 200 native fern species are also occur indigenously somewhere else. The strongest affinity is with south-east Australia, with c. 90 species in common.
This close relationship is reinforced by DNA analyses. The DNA of many New Zealand ferns is so similar to the DNA of overseas plants that there must have been dispersal across the seas surrounding New Zealand.
Even after it separated from Gondwana and became surrounded by ocean, New Zealand continued to exchange ferns with other landmasses, with both immigration and emigration. Ferns reproduce by spores, and these are so small (c. 0.05 mm) that they are comparatively easily dispersed by wind.
Brownsey PJ (2001) New Zealand’s pteridophyte flora – plants of ancient lineage but recent arrival? Brittonia 53, 284–303.
Perrie LR, Brownsey PJ (2007) Molecular evidence for long-distance dispersal in the New Zealand pteridophyte flora. Journal of Biogeography 34: 2028-2038.