In descriptions of New Zealand forests distinction is usually made between the broad classes: (a) Mixed broadleaf (almost entirely evergreen) types; (b) Mixed broadleaf forest (the same forest as above) containing coniferous trees, mainly podocarps (Podocarpus, Dacrydium and Phyllocladus) and kauri (Agathis australis); (c) Southern beech (Nothofagus) evergreen types; and (d) Transition forest between the southern beech and the other two.

In their primitive condition all these forests, though to a lesser extent the beech forests, are noted for the luxuriance of their growth; in particular, the forest floor is densely covered with ferns, mosses, liverworts, grasses, sedges, and many small flowering plants, and numerous tree seedlings. The trees and shrubs are in many tiers. Coniferous trees usually tower above broadleaf trees as scattered emergents, and only under certain conditions tend to form closed canopies themselves. The broadleaf trees below them form an irregular canopy, and below them again, growing under dense shade, are smaller trees, shrubs, and tree ferns. Epiphytes of Astelia, large grasslike plants, or seedlings of a number of families, grow in the crowns of the trees; and lianes of many species add to the tangle of vegetation. The canopy of the southern beech forests is more regular, consisting as it does of usually one or two species of beech only, and the trees, shrubs, and ferns below and the ground cover are all more sparse.

In their structure, the broadleaf forests are akin to wet tropical forest, and have been called by some ecologists “subtropical evergreen forest”. The beech forest, on the other hand, because it was thought that the origin of Nothofagus was in the antarctic (from fossil evidence), was called “subantarctic rain forest”. With the growing knowledge of Pacific vegetation and the reconstruction from pollen records of past vegetation movements associated with climatic changes, these concepts are, however, changing rapidly.

New Zealand lies between the latitudes 34°S and 47°S. Within the same latitudes in the Northern Hemisphere lie very large land masses. These are noted for once having carried very extensive forest of coniferous trees, pines, spruces, and firs, etc., and deciduous broadleaf trees, oaks, beeches, maples, birches, poplars, etc. The New Zealand forests bear almost no resemblance to these Northern Hemisphere forests, but they do bear a relationship to some Southern Hemisphere and high-altitude tropical forests. Although New Zealand is thought to have been isolated from other large land masses for a very long period, possibly since Triassic-Jurassic times, the same type of forest is to be found in other places throughout the Southern Hemisphere. New Zealand's isolation has, however, resulted in a high degree of endemism amongst species.