To understand New Zealand's present fauna and flora one must delve for the necessary background into the realms of geological science.

New Zealand is not ancient by comparison with the vast antiquity credited to the continental masses; nevertheless, there is irrefutable evidence in our rocks that our region has an antiquity back to at least 600 million years, that is, to the Cambrian, that world-wide base line marking the first appearance in geologic strata of well preserved and varied animal remains that constitute a faunal assemblage.

In visualising New Zealand's past history, one is prone to think in terms of this country's present size, shape, and approximate topography, but all three are very recent and very transient features. We must, therefore, project our minds back in time and become accustomed to the idea of a fluctuating land mass sited around a place on the globe marked by the present location of our country. We must envisage a land of continental dimensions at one stage, a mere scattering of small islands, an archipelago, at others – a land not entirely coincident with our present location but sometimes extending greatly both westward and northward. The actual site of our country as we now know it was a trough in the ocean bed which steadily accumulated sediments that were slowly built up, and in time became raised to the surface. Their contained fossil remains now give us the clues to our past history.

Another factor we must consider is that of climate, for the fossil faunas reveal fluctuations ranging from near tropical to conditions far colder than those experienced today. For instance, Conus shells, characteristic of warm seas, lived here in early Tertiary times but are absent from our seas of today – coconut palms flourished in North Auckland in comparatively recent times but disappeared during the world-wide ice ages, when New Zealand, although it escaped many of the rigours of northern lands, did see a great advance of glaciation, particularly in the South Island. Despite all these changes, New Zealand has existed as a more or less separate entity for a very long time, at least as far back as the early Tertiary, 50 million years or more ago, and during that time it has evidently had no direct connection with the Australian continent or with other southern lands. Nevertheless, there is evidence of the arrival of new colonising elements from time to time but not of necessity those which required a continuous land connection for the purpose.

One of the main colonising agencies at the present time is the East Australian warm water current which originates in the New Caledonian area, sweeps down the New South Wales coast, and then across the Tasman in a great arc, bringing with it not only adult long ranging swimmers that are induced to travel southward of their normal haunts, but also larval forms of marine organisms, many of which ultimately become acclimatised.

Examples of the long ranging swimming animals that reach our shores as adults are the swordfish, three species of marlin, several members of the tuna family, smaller fishes common in East Australian waters, two species of turtles, and two sea-snakes.