The distribution of rainfall is mainly controlled by mountain features, and the highest rainfalls occur where the mountains are exposed to the direct sweep of the westerly and north-westerly winds. The mean rainfall figures range from as little as 13 in. in a small area of Central Otago to over 300 in. in the Southern Alps. The average for the whole country is high, but for the greater part it lies between 25 and 60 in., a range regarded as favourable for plant growth in the temperate zone. The only areas with under 25 in. are found in the South Island, to the east of the main ranges. These include most of Central and North Otago, and South Canterbury. In the North Island the driest areas are southern Hawke's Bay, Wairarapa, and Manawatu, where the average rainfall is 30–40 in. a year. Of the remainder, much valuable farm land, chiefly in northern Taranaki and Northland, has upwards of 60 in. Over a sizable area of both Islands rainfall exceeds 100 in. a year but, with the exception of Westland, this is mountainous and unoccupied, much of it being forest covered.
For a large part of the country the rainfall is spread evenly through the year, although its effectiveness in summer is, of course, much reduced. The greatest contrast is found in the north, where winter has almost twice as much rain as summer. This predominance of winter rainfall diminishes southwards. It is still discernible over the northern part of the South Island, but over the southern half winter is the season with least rainfall, and a definite summer maximum is found inland due to the effect of convectional showers. The rainfall is also influenced by seasonal variations in the strength of the westerly winds. Spring rainfall is increased in and west of the ranges as the westerlies rise to their maximum about October, while a complementary decrease occurs at the same time in the lee of the ranges. The depletion of soil moisture through evaporation and transpiration is such that vigorous pasture growth is not maintained during summer unless the rainfall is at least 4 or 5 in. a month. Summer rainfall in most farming districts is less than this, and east of the South Island ranges it amounts to less than 3 in. in half the summer months. There, the dry summer conditions favour the ripening of grain and fruit crops, but farm production of all types is considerably increased by irrigation.
Areas which are exposed to the west and south-west experience much showery weather, and rain falls on roughly half the days of the year. Over most of the North Island rain can be measured on at least 150 days a year, except to the east of the ranges where there are, in places, fewer than 125 rain days. Those areas of the South Island with annual rainfall under 25 in. generally have about 100 rain days a year. In the far south the frequency of rain increases sharply; in Stewart Island and Fiordland rain days exceed 200 a year. Over most of the country between 55 and 65 per cent of the rain days also qualify as wet days (at least 0.10 in. of rain). The percentage increases to over 70 in Westland, but in the lowrainfall area of inland Otago there are only 40 wet days a year compared with 100 rain days. On the whole the seasonal rainfall does not vary greatly from year to year, the reliability in spring being particularly advantageous for agricultural purposes. It is least reliable in late summer and autumn when very dry conditions may develop east of the ranges, particularly in Hawke's Bay.
The highest daily rainfall on record is 22 in., which occurred at Milford Sound where the mean annual rainfall is 250 in. Other areas with considerably lower rainfall are also subject to very heavy daily falls; such areas are to be found in northern Hawke's Bay and in north-eastern districts of the Auckland Province. By contrast, in the Manawatu district and in Otago and Southland daily falls reaching 3 in. are very rare.
A century of annual rainfall totals at the four main cities is represented in the diagram above. Some adjustments have been made, where necessary, to take account of local changes in the site or exposure of the rain gauge. It may readily be seen that the pattern of variation from year to year is very erratic and shows no regular cycles or general upward or downward trend; and that abnormally wet (or dry) years do not usually occur simultaneously at all places.