A regional description of the geology of New Zealand, arranged by land districts, is presented later in this article; but first a summary must be given of what geologists have learned about the geological history of this country — the long sequence of events that has formed New Zealand's rocks and given them their present complex arrangement in the crust.

Events in this history are classified in accordance with the International Geological Time Scale. In this time scale are listed the eras and periods into which geologists have divided the earth's history, with each division representing an enormous span of time. This broad sequence was established in Europe during the nineteenth century. In the light of recent knowledge, the approximate ages of the eras and periods have been assessed in terms of millions of years. This has been achieved by studying the decay of radioactive minerals in certain igneous rocks whose relative position in the time scale was known from stratigraphic evidence. The Cretaceous period, for example, is thought to have lasted some 70 million years: the Paleozoic era may represent more than 250 million years.

Fossils are the keys that make it possible to equate the time of events in the geological history of one country with those of another. In countries like New Zealand which are far from Europe, it is not often possible to match fossils precisely; evolution, though similar, was not exactly parallel the world over. It has therefore been necessary to set up a local time scale, based on the sequence of rocks and fossils preserved in New Zealand, and to key this to the international scale at points where the fossils match closely.

The study of fossils has shown that the South Island has the oldest rocks; further, that rocks of all geological periods since the Cambrian are present in New Zealand, with the exception of the Silurian and Carboniferous. These, however, may in time be recognised here; it is almost certain that some rocks in Nelson are of Silurian age.

The eras and periods are not listed on the legend of the geological maps; instead, broader categories, such as “old sedimentary rocks” are given. Their approximate equivalence with the International Time Scale is as follows:

Alluvium, moraine, sand dunes, etc.: Holocene to mid-Pleistocene.

Young sedimentary rocks: Early Pleistocene to late Cretaceous.

Young volcanic rocks: Cenozoic.

Old sedimentary rocks: Mid-Cretaceous to possibly Precambrian.

Old volcanic and associated intrusive rocks: Cretaceous to possibly Precambrian.

Metamorphic rocks: Mesozoic to possibly Precambrian.

Granitic rocks: Mesozoic to possibly Precambrian.

The major divisions of New Zealand's geological history do not exactly correspond to the eras and periods of the International Time Scale. For simplicity, however, a division of the history into Precambrian and Paleozoic, Mesozoic, and Cenozoic is followed in this article.