Although relatively small in area, New Zealand is long and narrow, spreading over 13 degrees of latitude. This alone accounts for a wide variety of species, many with a restricted range determined by water temperatures. The fact that New Zealand straddles two great and distinctive water masses, the subtropical and the sub-Antarctic, makes for relatively sharp faunal distinctions between north and south. Other factors – ocean currents, both warm and cold, coupled with geographic and physical features – make it relatively easy to divide the shallow water faunas into five distinct marine faunal provinces. These, with their salient characteristics, are as follows:

Aupourian

Aupourian, Northland to East Cape and on the west coast, a division of some elasticity between Ahipara and Manukau Heads. It lies within the subtropical zone of surface waters and is even influenced by extropical warm currents, notably the East Australian Current which originates in the vicinity of New Caledonia, sweeps down the East Australian coast and thence in an upwardly deflected arc across the Tasman to New Zealand.

This current is responsible for occasional and even regular wanderings of certain larger marine animals to our shores. Instances are a number of species of fish, two species of sea-snakes, and occasional turtles. The East Australian Current also functions effectively in bringing to our shores, in their minute larval form, many species of invertebrates, particularly shellfish. The majority of these have become acclimatised and have thus introduced an element characteristic of warmer waters. Conversely, limited upwelling of cold water of sub-Antarctic origin on the Northland west coast brings in a small number of organisms characteristic of colder seas.

Cookian

Cookian, southern part of the North Island and northern part of the South Island. This is a mixed zone of subtropical and sub-Antarctic waters and supports a mixed fauna from these two zones plus sufficient regionally distinct species to make the area recognisable as a marine province.

Forsterian

Forsterian, lower part of the South Island, Stewart Island, and the Snares Islands. This province is greatly influenced by sub-Antarctic waters and in particular by the active West Wind Drift, which sweeps the Southern Ocean. The province is made up of a high percentage of restricted species and many stragglers from the southern islands.

Moriorian

Moriorian, the Chatham Islands, which lie over 400 miles almost due east of Banks Peninsula. The fauna owing to long isolation, contains a high percentage of endemic species and is particularly noted for the absence of a number of species common on the mainland. In general, the fauna is most like that of the Cookian but there are northern and southern influences as well, the latter the more marked.

Antipodean

Antipodean, the southern islands of New Zealand, Antipodes, Bounty, Auckland, and Campbell Islands. These groups lie well within the zone of sub-Antarctic waters. The fauna is composed largely of the more temperature-tolerant mainland species and the endemic forms of these, with a strong admixture of restricted sub-Antarctic genera. It is an impoverished fauna suggesting many extinctions in the past, due to adverse conditions.